Ehhh, I don't know. I do love he attitude, but what I see in her hind end is what my old vet called "knifey" movement in her hocks - she brings her hind leg abruptly up, rather than smoothly bringing forward under her body. Watch her move and you'll that her hind legs have a completely different movement and flight path than her fronts.
This is not what you want in a dressage of any level. OTTBs tend to be a challenge to school correctly in dressage anyway, because of the difficulty in getting them to relax their backs and move correctly back to front, this girl will have some additional challenges.
It will also really limit her scope over fences - 2'6" to 3" should be fine, I think higher will be a struggle for her.
I am quite sure chiro and massage would help her, but please ask yourself if you'd rather pay more money for a sounder horse or less money for an ongoing maintenence problem. If you really just want a pleasure horse and something to lark over low fences, I think she'd be fine, and I commend her sellers for being realistic and straightforward about her prospects. I hope she does find a nice home.
But, if it were my money, I'd pass. There are a lot of free and cheap horses on the market right now, and tons of OTTBs. I would look for something with less potential to be expensive to maintain.