Having just purchased a then, 6 year old OTTB in February, that was a BCS of scant 4 I think you have to trust your gut. So far my story has been a success story BUT I only wanted a trail horse that could do some simple and basic dressage. I highly doubt I will ever show him and really wanted a brain, which is what my vet assured me I was purchasing. He however would not guarantee anything beyond a good brain in my horse.
When diet was discussed the reason he was a rail was obvious, the seller was only feeding him 3lbs of feed daily. His ration was 1.5lbs of rice bran and 1.5lbs of alfalfa pellets and a very crappy fescue round bale. He had no shelter and a constant rotation of pasture mates, one that was a cryptorchid that bossed him constantly. Now on 8lbs TC Sr., 1lb alfala pellets, .5lb flax, G.U.T., and Smartpak Muscle Mass daily with 5-6lbs of bermuda hay he is a solid BCS of 6 and the sweetest easy going horse.
I took a chance because my gut told me he would be a friend and that is all I wanted. I also had a budget of 2k and very little expectations. With your budget and as alluded to previous experiences I think I would pass on the TB mare. As others have stated you could pick up a green WB for that price or at least a proven TB. Just my two cents worth