If you think that horse sounds wacko you would think mine were criminally insane...and they're lesson and higher calibre show horses. It sounds like he was ridden by someone that couldn't handle him and he was fighting back the only way he knew how, I.e being reactive and popping a few bucks. That's pretty easy to fix if you know what you're doing. The injections would be a hang-up for me but most big barns these days inject them just because they can not always because they have to. He is free so I would definiely spend the money for a good PPE but someone lacking funds that is looking for a decent jumper he would be a steal if he vets even relatively clean. And some gymnastics and dressage would snap those knees up, just because he hung them this time doesn't mean he can't get snappy. That just takes correct training, good riding and time.
Not sure if I am reading the same description as everyone else but he doesn't sound like a wacko at all. Just needs some confident riding. Bucking after jumps is usually a learned habit, picked up from bad riding when the rider lands hard on their backs or jabs them in the mouth. Or both. I'd buck too.
I guess each person has their own definitions of wacko. To me he actually sounds more on the sane end of the spectrum.
Joint issues are a deal breaker for me, I wouldn't touch him with a ten foot pole because of that. Too bad he sounds like a cool horse.