If I'm not mistaken, you're talking about non-indexed derailers, plus the chain width would be the same for 5 and 6/7, so there should be no problem.
Installing a derailer is a fairly simple task, in and of itself. But you either have to break the chain or disassemble the derailer to get the chain threaded through it, and that's a little more involved. Plus you need to adjust the derailer once installed (though adjusting a non-indexed derailer is almost trivial).
None of this is beyond the abilities of someone with some mechanical ability and a modicum of patience, but it requires thought, care, and a few tools. And note that you'll maybe want to replace the shift cable at the same time, as the existing is likely too frayed to reuse.