At 7 I'd say almost always this wouldn't be acceptable.  At 15-20, it might be depending on what the vet check results were.

You need to have a good equine vet (not just a vet that can see horses) to inspect this horse from top to toe.  Sounds like X-rays or a fluoroscope on those rear legs would be in order.

Having said all this, I recommend buyers plan on spending as much on their vet check as makes them comfortable.  What seems to be enough for a $25,000 horse's exam might be too much to spend on a $1,500 horse's exam.  Do what you feel needs to be done in your price range.  (An $80 set of X-rays might save you thousands in injections over time...a good vet will play the pro/con game with you and explain everything.  The vet needs to know what you will do with the horse and what you would be willing to treat and what is a deal breaker.  I always test horses for drugs and do cultures on mares no matter what their purpose.)

I've had my share of old tired animals(dogs,cats and horses) that have been entrusted to me, and I've done a good job providing quality care. Expert, not in my opinion, with my education and 30+ years experience, when it comes to equine health and nutrition I'm very knowledgeable with plenty of hands on know-how.

 PS : notfromtexas, I might take an advil before seeing a doctor. But phenlybutazone, unlike advil, is not an OTC drug.

I can get bute without going to the vet's office, so for me it is OTC.

It also works really well on back pain for humans.

"Contrary to popular opinion ‘Bute’ is one of the safest drugs on the market today. Literally millions of doses are administered to horses every year – adverse reactions when ‘bute’ is used within normal dose rates are virtually unheard of. When used over a prolonged period (i.e. years) ‘Bute’ can cause mild damage to the liver although this is uncommon. For any horses on a repeat prescription e.g. for chronic lameness, it would be a good idea to do an annual blood test. ‘Bute’ – more correctly called phenylbutazone is one of the most commonly prescribed medicines used in equine practice. It belongs to the group of drugs that are known as Non-Steroidal Anti-Inflammatory Drugs (NSAIDs). This is the same class that includes Ibuprofen and Aspirin. Phenylbutazone is primarily used in horses for its anti-inflammatory and analgesic effects. It is particularly useful for the treatment of orthopaedic pain i.e. lameness, but is by no means confined to this. " (Pool House vets website)

I'm aware that it's a controlled substance, hconley.  At one point, so was ibuprofen and naproxen.  At one point, it was legal to buy cocaine and heroin, too.  My point was that a good horse person should be able to give  a slightly stiff or sore horse a little help for a day or two. 

I can get bute without going to the vet's office, so for me it is OTC.

How?  I give my old girl bute before the fairrer comes out  because she has arthritis in her hips and getting those back feet up high enough for a trim is tough on her.  The vet here doesn't think she needs it and I'm almost out.