Congratulations!!! That's a badass ring :D You're clearly already thinking about this logically--venue and invites are absolutely the first things you need to do. As for how long you have to send them out, it really depends on your guests. Are most people local, or will most people have to travel? The more people that have to travel, the more warning you need to give. Even if you don't want to do save the dates ($$$), I'd probably start getting the word out as SOON as you have the details confirmed, especially to out-of-towners. Be prepared to field questions about the state of your womb... Sigh.
The rest should be much easier to do non-traditionally ;) Yes, most wedding caterers are booked 18 months in advance, but not food trucks or Chipotle. You probably can't order a custom designer dress, but luckily it sounds like that's not what you want anyway, and you can find amazing dresses through tons of unusual avenues. Screw DJs if you can't find one for your date--I've been to self-DJ-ed weddings that were WAY more fun than anything with a pro. (One was actually on a boat in Puget Sound, come to think of it!) There are SO many alternatives to traditional florists, even if you decide you want fresh flowers. Online delivery, grocery store florists, pick-your-own flower farms... or you can make flowers out of different materials, or just do decorations that have nothing to do with flowers whatsoever. Other vendors often don't get booked until closer to the wedding date anyway, like ceremony musicians and hair and makeup people, so you have more time to think about it. Finding an officiant shouldn't be TOO tricky--regular ones may be booked, but they can usually do more than one ceremony in a day. Or just get someone you love ordained, although make sure you give them enough time for the certification to come through.
Speaking of which! You should take advantage of your friends' talents, as long as they're freely given. The other winner for most fun wedding I've ever been to featured a funk band composed entirely of friends of the couple--the groom even sat in for a few songs. These were mostly gawky white guys who usually write academic classical music, and it was AMAZING to see them let their hair down :D
The thing that's probably hardest to DIY or do non-traditionally is photography. If you want pro photos, you'll probably have to deal with pro wedding photographers, end of story. I'd think that might be the hardest vendor to find.
Good luck!!! This is gonna be fun :D