You will most likely find the best information in the "transmissions" forum.
I would say rebuilding the diff is likely to be cheaper in parts, if you are confident you know and can replace what is wrong. If it has to be professionally rebuilt, I would say sourcing a junk yard replacement is likely cheaper.
Cheap and good performing don't generally go in the same sentance. The clutches are not cheap for these cars. If you have never done a clutch before, you will still have the dual mass flywheel which is generally not reused.
What year WRX is the clutch from? You may be able to buy a WRX single mass flywheel to match the clutch, and run that matched pair. Be sure to replace the throwout bearing, and check for any damage on the transmission "snout."
Personally, I got a 07+ LGT single mass flywheel and clutch kit for ~650ish from fred beans parts.