Why do you want to buy any old bike. I suggest you buy a new one, at Committee Chowk RWP I saw some light ones for Rs.5,500/=. These were without any shocks (less problems) and the build was reasonable.
If you want to buy old one then visit China market, F-6/4, and be astonished by the ridiculous prices. But, you may get lucky and find what you need!!!