This is what I was worried about, and was asking about here. I would think he would be much happier peanut rolling and collection work would be hard.
And you were absolutely correct
I just wanted add a couple of thoughts for your own future reference as you continue on your search: Horses that are slightly downhill and/or have a low set neck can
be capable of lower levels in dressage with the right training and conditioning. But you really must take into account the whole horse to see if faults such as these can somehow be balanced or compensated for.
True collection isn't seen in the lower levels so the most critical points to consider are correctness of gaits (impulsion, rhythym, freedom, straightness etc), accuracy of test (related to submission, acceptance of aides etc), willing and balanced transistions of course your own riding (position, balance etc). Many of these criteria are really contingent upon having not just a horse that is balanced overall, but particularly with hindquarters and hind leg structure that will bear up under this type of work.
The deal breaker for this guy IMO is not the fact that his neck ties in low or his downhill build. While certainly not optimal, it is the fact that there is no chance in the world that you could improve upon those things given the weakness of his hind end. In dressage there is great emphasis on strengthening and using the hind end so that it can be active and really drive each gait and the transitions between gaits.
I just thought I would add this in so that you can have more of a goal in mind when you look at horses and an idea of what can possibly be worked with, and what cannot.
I think HowClever said it best in relation to this guy:
Holy dodgy rear end!
That said, best of luck with your search!