I bought my boy 2 years ago, knowing he had had thrush in the past and he got it again last year. He has never been lame with it and is easily treated, farrier comes out to sort it out, then i cleaned and iodined until it was gone - he just has such big, deep feet that so much mud gets in there. This year I have pre-empively iodined daily, and nothing so far, touch wood.
If the worst point of the horse is it has had thrush, I would count yourself lucky!
Yes, pevious horse suffered as had deep feet. Regime just became 2nd nature as in picking out feet, brushing out and then treating with pevodine solution twice a week. If really bad can get foot rot spray from vets which is great. Go fo it.
in the majority of cases, thrush is a management problem. scrub feet out on a regular basis with either .. salt water, hibiscrub, iodine, pevidine, hydrogen peroxide, nappisan. milton,etc. keep bedding clean and dry. keep the feet regularly trimmed. prevention is better than cure. No it wouldnt put me off buying the horse, if he fits the bill.