Pro trainer here. I've trained horses in 5 countries with and for Olympic and Grand Prix riders. But to be honest I only skipped around in the video because it's 14 minutes long!!! Yikes. Ain't nobody got time fo that. I'm also not sure if you mean the discipline English pleasure or you intend to ride her English but not show. So take my advice as you will.
Pass on this horse. Here's why from my standpoint:
More important than anything else she's a Rocky Mountain horse and she's gaited (confirmed in video). For that alone she's unsuitable for English pleasure and most/all other English disciplines. The end. Getting a horse who can't do what you want is a recipe for heart ache. And like many gaited horses she's beautiful but not built for English pleasure; high set neck, long back, weak coupled, downhill build.
She seems more firey than they imply in the ad/video or has some anxiety towards riding. I certainly wouldn't put a child on her. She's going as quick as she can and only the big bit is restraining her-- a bit you can't use in English pleasure, and English pleasure looks for calm, relaxed horses who look like they could fall asleep at the canter. Assume any sale video shows a horse at their absolute best. She strikes me as a horse who could become unruly or dangerous for a novice. I've seen too many novices fall for a "pretty face" and end up terrified of their horse or suffering anxiety. Just because they can control her doesn't mean you necessarily can and feel safe (not to imply you couldn't either).
There's concerning holes in her training. She doesn't travel straight or even keep her head straight. Her transitions are sloppy and she doesn't maintain a steady pace. She doesn't know how to use her body or bend anything except her neck. She's being ridden entirely off the bit and strikes me as a counterfeit horse that's been "trained" (read: forced to do things) through over bitting and other shortcuts. Horses like that are anxious mounts who get frightened when they think they did something wrong or when they're confused. The steering is poor and the obvious mistakes and sloppiness show the horse is just guessing what to do and they're acting like it was their plan. I don't want to see a horse wander around randomly I want to know the horse is definitely following the rider's plan.
Gimmicks in the video and "buzz words" in the ad raise red flags. I guess it's impressive you can sit backwards on the horse, but why do I care? I don't ride my horse's backwards, and it tells me the horse wasn't trained to go off seat and leg cues because you're just using your hands. I could retire to Hawaii right now if I had a dollar for every ad I've seen with someone standing on the back of a horse who's actually poorly trained and unruly. I want to see the quality of training, not just desensitization; those are two different things. I have a 6 month old mule filly right now I can throw a tarp over and lead around through the wood... doesn't mean she's trained to a high level, she's just very desensitized.
Despite the high talk of their "Signature Horses which they never sell" the presentation and riding in the video are unimpressive and unprofessional and I doubt they're professional trainers. While an amateur can certainly train a horse well, the riding in the video doesn't strike me as someone who can produce a solid, correctly trained horse, and indeed the horse doesn't appear solid at all. A horse can't be trained any higher than the level of the person training it. Whats more, if they truly never sell their horses then I wonder why they're selling this one? Maybe they just need he money but it's pretty much always the worst saleable horse you sell first if you're not in the business of selling your horses. Obviously you can't be suspicious of everyone who's selling a horse or you'd never buy one, but I get the distinct impression these people are over-representing themselves. Just the way they did the video alone, with sloppy text that should go in the ad not the video and with it being 14 minutes long!!! etc, let's me know they're not real professionals.
With your budget you should be able to find the perfect horse to suit you. This horse isn't worth that price. She's pretty, but remember: pretty is as pretty does. I'm not necessarily saying don't go look at her, but I do think It'd be in your best interests to not buy her or at least be extremely observant and wary while you're there and trust your instincts. If possible bring a horse professional with you. Sorry this is getting long, but here's some tip when you see her, because I really think you're making a mistake with this horse:
Evaluate if the owners and the facillity live up to the picture they're painting or if they seem like backyard trainers. Are the stalls clean, or at least have only 1 day of manure in them? Are all the horses in good condition, with well maintained feet and glossy coats? Do they seem happy? Does the barn seem like a horse paradise or a rickety, dirty horse prison?
Be cognizant of any pressure you feel or any excuses they make, like: "I don't know why she's being bad, she's never been like this before! She must just be nervous today because of X reason" or "Of course you can rider her in an loose ring snaffle, we do it all the time! That's how we usually work her. I'd show you but I lent that bit to a friend yesterday."
Always have the owners ride the horse first, and think up a pattern (or a simple Training level dressage test) involving circles, straight lines, a figure of 8, transitions up and down, and shows all gaits to test the obedience, responsiveness, and straightness of the horse.
Be aware of anything like a saddle mark on the horse or if she's wet as if she was just hosed off. Some people will ride a fresh horse before a buyer comes until the horse is totally spent so it'll behave better for the buyer. Feel the bottom of the saddle pad for wet sweat and check the bit for fresh spit or grass that isn't dry.
If you show up and the horse is tacked up and ready to go, ask them to take the tack off and turn the horse out before anyone rides it. Demand it if you have to. Make sure they can walk up to the horse, halter it, lead it to the barn, tie or cross tie it, pick out its feet, brush it all over, and tack it up. Trust me on this.
Show up an hour before you're scheduled to get there. Catch them by surprise and see what they're doing-- say you're sorry, you left a little early in case you got lost and the drive ended up being shorter than expected. You can learn so much you otherwise wouldn't by doing this. Are they riding the horse or evidence she was ridden? Even if they're working with another horse it gives you the chance to see how your horse was trained; if the horse has its head tied down between it's front legs and a bit the size of a Chevy in its mouth, or if a horse is tied to a tree while they terrorize it with scary things to desensitize it, I promise you your horse was trained that way too.
Especially note if there's a med kit out, bottle of injectable drugs (peak at the label if you can), or a fresh syringe sitting out. Look very closely on both sides of the neck and under the mane for a speck or smear of blood, especially over the veins. You'd be surprised how common it is to ace a horse for the prepurchase, especially if the vet check isn't that day. If it's not and it can't be done the next day, get a tube for blood from your vet and ask them how to keep the blood viable and bring it to them after you see the horse. If you have any suspicion about drugs, either you or them (in front of you) collect blood from the horse for drug screening. If they don't know how to do that they're not professionals. Signs of drugging include a hanging lower lip, droopy eyes, low head, floppy ears, sweating even though the horse shouldn't be, and/or a dull reaction to simuli. Be aware a horse can be drugged and show none of those signs.
Watch the video before you go out. Watch how she reacts in the video and holds herself. Look at the tack she's in etc etc. You want to make sure she behaves the same way as in the video and isn't put in a harsher bit when you're there. If she seems quieter when you're there that's a red flag.
Final tip: bring your English tack and don't warn them. Ask them to rider her in it at the end. Watch how she reacts to the new saddle and if your bit is sufficient. If you can't do that for some reason bring something that's kinda scary but not crazy, like a crinkly feed sack, and ask them to put it on the horse's back. If it's really totally bomb proof and beginner safe and kid safe, it should be fine and not even notice. If she's snorting and rolling her eyes at it or frightened, then she's not very bomb proof. Also bring one of those compact umbrellas where you push a button and it springs open. With her loose, walk up to her and open the umbrella suddenly in front of her. I wouldn't fault any horse for startling at that or even moving away, but if she blows up or bolts away in a panic or freaks out or rears up you know that's how she could reach to something unexpected with you on her back.
Sorry, this got super super long. I'm just concerned.