This is one thing that really bothers me. I know the horse is being sold for a reason. Perfect horses do not normally just get sold. So why are they selling it? Most ads make them all out to be those "perfect unicorns" mentioned. I'm trying to figure out before I even go to look at the horse with a trainer or mentor, what can I ask to determine if that unicorn is really just a fire breathing dragon in disguise. I'm honestly thinking I would be ok with a nice tempered pasture pet, but the more I talk to DH about my upcoming riding lessons in July the more that may change. I'm already having issues with people, but to say I don't trust sellers, well, that would be an understatement haha.
I've bought two horses in the last three years. We lucked out on one, not so much on the other (but decided to keep her and restart her from the ground). Ask a ton of questions. You have a lot of great suggestions above. But also, ask why they are selling the horse. If the seller is reluctant to answer each and every one, forget about this horse, and move on. If you are satisfied with the answers, ride the horse. Not just once, but at least twice, on different days, and in different scenarios (ie, one trail ride, one ride in an arena). Bring someone with you on at least one of those rides. Get the owner to ride the horse first, then you get on. Maybe get your trainer to ride it too. A horse that one person thinks is great may not suit your needs. I rode a horse that was bombproof and beginner-safe, and found her boring as heck.
The PPE is great, but do ask the seller if the horse has had any health issues that they are aware of, and have a detailed contract for the purchase that includes such wording, asking for the seller to detail any health issues. This will protect you somewhat if they're lying, and more importantly, they'll be less likely to lie in writing than verbally. Do a trial if you can, or ask about a buy-back period (this is what we did with our first horse - we had two weeks to decide if we were keeping him). The buy-back contract can have a clause saying that if the horse is injured during the buy-back period, it is no longer returnable. All this stuff is hard to prove, but it goes a long way towards making both parties feel more secure. If they won't do a trial, ask to ride the horse again, and again. Visit the horse when they aren't expecting you. Arrive half an hour early.
Even if you are ok with not riding (though you seem unsure about this), make sure the horse is easy to handle. Ask to catch him in the field yourself, or at least watch someone doing so. Lead the horse around the property so see what his manners are like. Groom him, pick his feet, tack him up. Is he a biter? a kicker? Does he have any vices?