I probably wouldn't buy a horse with a sway back. He's relatively young and the sway will likely get worse as he ages. Saddle fitting will be a nightmare (I wonder if the saddle being used on him now actually fits....somehow, I suspect it doesn't).
If he's perfect for you and you really want to bring him home, go ahead. BUT, you need to be aware that finding properly fitting tack for a horse with even a slight sway (as Wallaby discussed) can be terribly difficult. His back will be weaker so he won't be able to carry as much weight or be ridden for as long as horses with a good back.
He'll be relatively expensive to care for between the tack and, I'd have a chiro out to do some pretty routine work on him. Also, you never really know how long he'll actually be rideable. He might be perfectly fine under saddle for another 15 years.....or he could be broken down to the point of being unrideable in 5 months.