The clicking sound from the gears almost always points to a rear derailler cable that's a little loose. It could also be an out of alignment rear derailler hanger. Either is a relatively simple fix. The above mentioned websites will both have good info on how to go about your repairs.

I'd say you got a decent deal on the bike especially given the age. All of the bikes I've bought for under $100 have been 10 years or more older.