There is, unfortunately, one additional thing to think about before becoming the local 'I'll fix your junker bike up' guy.
A lot of these gems were barely safe enough to ride when they were brand new, with questionable brakes, etc. Should you decide to bring back one of the rusty, previously non-maintained beauties to a state where someone would actually not be too embarrassed to be seen riding it, and something go very badly (that brake cable you didn't replace suddenly snaps, causing the rider - who most likely isn't wearing a helmet - to go butt over teakettle after striking the object he/she was trying to stop for) don't be surprised if the same person who thought you should be nominated for the Nobel prize for 'fixing' the bike suddenly decides you're the reason he/she has brain damage, and sues you for perpetual care.
"I see you riding your bike all the time, so you must know how to work on them, right?"
"I see you driving your car all the time, so you must know how to overhaul my automatic transmission, right?" Just direct them to the LBS. Usually the shop owner will give them an estimate that will solve the issue right then and there.