From what you describe it sounds like he's being more fearful (or in pain/confused, etc) than defiant. Sounds like he needs more patience and consistency for a starter. Usually, if a horse is like that, I ignore unwanted behavior (unless he's directly striking, biting, kicking at me- that's a whole different story) and just continue until the desired outcome (or semblance of it) is reached. (Way before any saddle).
It takes a long time. I don't really hold on to "once a rearer always a rearer" mindset- I think that's true for horses who have learned it as a defiant act to get someone to go away real quick, but seems like something else going on here (I could be wrong, of course)
If you get the guy, I would encourage you to be patient. Horses will take 1 step forward, 2 steps back, 1 step forward, 1 step back, 2 steps forward, 1 step back. And on and on... So don't get discouraged if things go backward after progress.
I got a guy now (whole host of other problems, boils down to he's fearful/aggressive, flips out and attacks with little pressure). Some days he does really well, other days its like he forgot absolutely everything and wigs out again- but all in all, we've made substantial progress from when he first came around, so I have to remind myself to not get discouraged when he regresses a bit. Just slow down, go back to a comfortable thing and try again. (Though I do make it a point to get him just outside his comfort zone, enough that he's uncomfortable, but not enough that he freaks out. Just so he can think how to react, not just react, I call it "Frequent Bouts of Mild Torture")
Those are my 2 cents. Good luck!
And.... He is absolutely gorgeous! I think you'll be alright so long as you know you're going in for the long haul. No quick fixes, no easy buttons. Just patience and persistence.