I would just decide whether or not you want the horse. If you offer anything significantly less than the accepted offer then the seller will most likely (and understandably) back out. If you only want a few hundred dollars less then in the grand scheme of a horse's life it's not worth arguing if you like the horse IMO. I would just decide whether or not you want to deal with this mare's potential issues, and either go with the current offer or decline.
You could talk about lowering the price, but I'd expect the owner to decline. The odds of finding an older horse with zero issues are pretty slim, but I don't know enough enough from him saying that her back end was "off" to say how much it would affect you.
Thanks Duck Dodgers, I finally got the copy of the vet report today after they had sent it to the wrong email address 2 days ago
Here are the points made by the vet:
-Altered movement in hindlegs on initial trot up (slight upward slop). Left hind reduced stride in trot, persisted when examined on flat lunge. No change with flexion. unknown cause and no palpable leg abnormalities.
-Intermittent left fore lameness with head nod on left rein lunge and on slight downward trot.
-Facial skin lesions with sarcoid characteristics, also found on body. Not currently irritated, not in tack areas. reported as being present for long term.
-Poor neck musculature and dipped crest.
Adding a few other things that I have further learnt from the owner - she suggested I lunge her before riding in spring time (not so keen on this) and also that the last owner said she had issues with pulling back while tied but she cant comment on this herself as she has always handled the horse in a stall.
I feel really torn but I think my head is telling me to pass on this one. I am going to ring the owner tonight, I don't think I will offer less money because while I might be prepared to accept and work on these issues for say $3000 like you said - I doubt she would accept this.