Can you get a vet out there to do a PPE (Pre Purchase Exam)? To be sure? I hate to say it, but people have been know to, er, stretch the truth somewhat in horse ads. Maybe you could even ask for the horse's vet records? I got mine AFTER I bought my first two horses, and it did contain some information that the owner had not seen fit to share with me. I find it hard to believe this person could not find a buyer for a good riding horse.
Oh, and another reason people give away horses: because of behvaior problems. Rearing, bucking, biting, kicking, etc. Could you handle those if they came up?
If this horse turned out to not be rideable for some reason, would you be OK with it just being a pasture pet?
Just, again, to reiterate, I hope you can make this work out! But you need to be 100% sure of what you are getting into. Well, at least 90% sure.