Personally, I would probably not buy the pony. I would definitely ask for more testing from the vet to determine the severity of the sidebone changes though, if I otherwise liked the pony.
From how it was explained to me, sidebone is when the cartilage on the side of the coffin bone is turned hard, basically into bone. It certainly is possible that the pony can go into full time, regular work - and even hard work - but there's too many ponies out there who don't need a health issue managed for me to buy one that does.
Just walk into the situation with open and educated eyes. Get a thorough examination of the sidebone status and consider getting a second opinion. Like I said, the pony may very well turn out to be fine for many years.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.