I know it's already been said, but I don't think it can be said enough. Stifle injuries can easily take 6 months or more to heal. Then you have to be very careful about how you recondition the horse. If he actually has a stifle injury your looking at no riding for quite awhile then walking in a straight line for longer then adding in a little bit of trotting and etc. So if you have any plans of really riding/using this horse in the next year you definitely need to re-evaluate getting him.
My horse had a very mild stifle injury early this spring (beginning of May I think) and he was laid off until about the middle of September. He's been ridden VERY lightly since then and I will be beginning a conditioning program in about a week. He should be good to go for normal riding this next spring.