If your shifters are friction, you can purchase a 7-speed cassette for $14

http://www.chainreactioncycles.com/M...?ModelID=47962

and a new set of wheels with alloy rims and stainless steel spokes for $56

http://www.chainreactioncycles.com/M...?ModelID=42536

Surely the hub bodies are alloy, right? Beats me--ask.

I have the impression that OP wants something quick, easy, inexpensive, and durable. If this doesn't do it, I give up.