I would want, for an eventer who could go upwards to **, at least three quarters thoroughbred. At least.
Also something with power in the hind end. That's where your dressage, and your jumping come from.
I also like something with a bit of gee up and go. Not something that half way around the dressage arena, it needs a tea break!
I like something with a good 8 or more inches of bone. 8 and a bit is ideal.
I want something that is a trier, and will jump it's heart out for you.
You don't want something that is really heavy on the forehand, say if it has a low set neck. Crap for dressage, and it'll never make the speed out cross country.
You also don't want a draft type horse. It will really tire it's self out, and hit the ground hard.
In terms of conformation, good legs, feet and pasterns are priority. Without those, you have nothing.
I don't mind too much about the withers... As long as you have a saddle to fit. My eventer has knife withers, typical of the TB, but his saddle fits, so it doesn't make a difference to me :)
A good shoulder is a very nice thing to have, particularly in the dressage and cross country phases. In dressage, to get that wow trot and those super movements, and out cross country, so he can extend out really nicely, and make up time.
I also want a high or medium set neck.
Straight movement of course. Dishing will never do :P
In terms of height, you really need something around about 16hh-16.2hh. Any bigger and the horse will trip over itself. (diddly is 16.1hh, and he has great difficulty in not tripping over his own feet!) The bigger the horse, the harder it is to keep it sound as well.
Anything below 16hh, unless it has an effortless gallop, and brilliant scope, will struggle quite a lot with the times and the fence heights out cross country.
Just for fun... My eventer :)