Hi Will....welcome to the Club
I would definitely bring it back when you return as it sounds like a superb car....2.8 and coupe!
1. my Z3 diff has always sounded a bit whiny but has always done that....you could rebuild it with new bearings, etc.
2. Sounds like a dynamic wheel imbalance....get the wheels balanced at any tyre bay.
3. Springs and Dampers are very cheap through GSF or Eurocarparts....would also recommend new suspension bushes as well.
4. GSF are pretty good at supplying quality parts at cheap prices (look out for regular discount codes online)
Keep us updated on how its progressing and would love to see a photo or two
Cheers, Trevor