I'm currently on the hunt for a pony eventer, and my main concerns are for them to have basic correct conformation with willing temperaments. It's hard to judge a horse completely on his suitability for a discipline just by a conformation shot, and there are also many horses that compete with what many people consider to be "major flaws".
It also depends on what level you hope to compete. For BN, really anything can do it. Novice, you have to be pickier because the demands are higher, but still most athletic horses have no problem jumping a 3' course. Buying anything unproven for anything above training level IMO is when you start taking a gamble, (mostly for the cross country) but hopefully if you wanted a prospect that could take you there and/or higher hopefully you'd have a trainer to help you first hand.
TBs are great eventers, but I'm not sure they're ideal for the higher levels solely for the dressage. Conformationally TBs are bred for speed and endurance, and aren't typically purchased as high level dressage mounts. There are always exceptions, but just saying.