Probably the least expensive/least hassle path is to do the 8-of-9-trick. A 7-speed body won't take a 8/9/10-speed cassette, but it will take a shortened 9-speed. Sheldon Brown will tell you all about it. So get yourself some Shimano compatible 9-speed shifters(microshift have gotten some favourable reviews WRT bang for the buck) a couple of downtube cable stops, a 9-speed cassette, new cables, new bar tape, and get to it.