Bar - we've been over this before, a long time ago. If you want to shell out thousands of pounds, then get a treadmill for respiration and heart functions, sigmoidoscopy, ocular, bloods and parasitology set from the vet before you buy. It'll take time to get the results back to you, by which time someone else will have taken a punt and bought the beast. If they find he's having a problem breathing, they'll then get the sigmoidoscopy (scope for short) and see if he's got polyps, an epiglottal flap, etc. But there's really no need to go this far if the horse looks well, moves well, and is reasonably proportionate physically.
If you buy something narrow and slabby, then it could still pass all the tests but never be a winner. And even if your chosen one passes them all, there is nothing to say it can't develop problems once in hard training. There's possibly a test around for checking bone density (I don't know if there is, but it'd be a damn good one), so you could determine the porosity of your horse's bones prior to setting a training programme - but I sure don't know of any trainer who bothers with one. True, some youngsters snap a brittle leg on the gallops - but that's just the, uh, breaks.