Depending on whether it is a Potenziata or not, it may be worth having the diff fixed.
The 75P had a 3.9:1 diff ratio which matched the slightly hotter cams and motronic much better than the extremely long 3.5:1 ratio in the regular 3.0
If it's a regular 3.0, I'd ditch the whole transaxle and put a TS box in it.
The car goes so much better with a 4.1:1 diff ratio, but you'll lose a bit of highway economy.
The changeover costs are hard to put a figure on- I think a TS box is worth about $1500.
Yours will probably get you about $500 if it's buggered and needs a rebuild, and you'll need a few hours labour.
If you're going to do timing belt, you may as well look at getting the water pump done while you're down there. I'd say about $1000 for the whole thing.
Any reputable shop will have an idea on costs - ring around and ask