As you can no doubt tell by now, converting to indexing means converting the better part of your drivetrain. Although there is something to be said for indexing, that something does not include "cross-manufacturer compatability" and "upgrade/maintanence ease".

A nice, long cage suntour vx or vgt or whatever is prolly the simplest, most headache free solution, although as many have noted, most any derailleur should do-- as long as you keep your system friction.
For decent range, you want a long cage mech.

Stronglight's advice re: derailleur hanger alignment, derailleur limit screws, etc. is excellent and I really have nothing to add.