Yes, the buyer pays for the PPE, but you might have to ask the vet to include the 5 panel test. It can take 2-3 weeks to get results back, so it might be reasonable to put down a deposit on the horse to hold him while you’re waiting. And yes, if the seller balks at the 5 panel test that would be a big red flag.
When you go to look at a horse try to find out as much as you can about his history–how many owners, why he’s for sale, what kind of riding has he been doing (trail, dressage, obstacles, cow work, etc.), who has been riding him (beginner, intermediate, etc.), his place in the herd (although that could change in his new herd), how he has been kept (24 hour turnout, stalled, dry lot, etc.), what he eats (grass, hay, grain, forage balancer), any special dietary needs (easy keeper/hard keeper) and anything else you can think of to help you get to know this horse.
Also look him over for any physical issues, and pay special attention to his feet. Does he require special shoes? Can he go barefoot? Long toes can be corrected, but founder rings are a red flag. If you find significant scars ask how he got them. Ask if he’s ever been lame or had a colic episode, and if the answer is yes delve deeper. Abscesses and stone bruises are no big deal unless they happen frequently, whereas chronic laminitis is a deal breaker. White spots on the withers can indicate he’s been wearing a saddle that doesn’t fit. Sores on the corners of his mouth can indicate rough handling or a bit that pinches. Most of these things aren’t deal breakers, but they can give you an indication of the life he’s had.
Yes you should get another horse. You had an awful experience with Jasper but nobody could have handled it better. You absolutely proved to all of us that you have what it takes to be a great horse person, and any horse would be so lucky to land with you.