good find. I'd take it. The clicking is probably the result of the derailleur being slightly out of alignment on some gears. Basically, the clicking is caused by your chain preparing to jump sprockets. If you have lever shifters, adjust them ever so slightly once you get to a gear you like. a little oiling would'nt hurt either. I don't think you have anything to worry about with the rust.
Also, 50lbs is really, really hard to carry on a rear rack. Putting some of that load in a backpack will take less stress directly off your back wheel, but when you start pumping up hills, it will get annoying.