I only ever worked directly for a single family, not with any contract agencies, so take this fwiw. I did it for about 10 years though so I've dealt with a lot that did however.

I'm former military with a noncombatant mos... So yea, being a spec ops type isn't strictly necessary. It would be very difficult without military or law enforcement backgrounds although I worked with several guys that were neither also. They did not have longevity though. Gavin de Becker's group is one of the larger employers, they have a lot of celebrity business, and simply won't consider non vets at all. They can be a good foot in the door if you are a vet though. With no experience you will probably be estate security to start with them from what I've seen.

I would recommend against pursuing the line of work. Unless you are lucky like I was and get on a permanent detail, it is very feast and famine. You are going to be competing against a lot of former or current LEO, former SS or DOS dignitary protection, and former military contractors these days. I've known several guys, even with combat experience, that have spent thousands on training with Blackwater and what not that are doing something else now. Work is only really available in high cost of living areas because that's where the rich people live. A lot of people I've met in the field are shady as fuck, it seriously is like moths to a flame. And then you have protectees that are just plain tortuous to deal with. And even if they are cool (mine were) they will have shithead family or associates that will drive you up,the wall. And they don't need you hardly as much during normal working hours, so kiss your weekends and evenings goodbye.

It's not all bad of course though. Flying to Europe on a private jet is pretty cool. We would usually sit up with the pilots in the jump seat. A couple of my coworkers went with the family on an African Safari. I can honestly say that I can do a reverse 180 in an armored SUV, although that hasn't gotten me laid yet.