The big question is fit. If the bike is the right size for you then go for it. Does it have a nice long top tube and im sure it has horizontal drops and fender and rack eyelets.

You can actually keep everything on the bike and just get a 27" rear track wheel.

http://www.totalbike.com/items/wheel...ing_s_7d38.htm

Maybe ask the guy if he will come down on the bike a little bit, but the original question of how bad and idea is this-- the answer is not very bad.