Congrats on a real nice system. IMHO, the 3000 was the pinnacle machine from Commodore.
For RTG go with a Picasso IV or a CV64. If you can live without a passthrough and use a switch a Retina BLT Z3 is a real fast contender also.
Cheap upgrades:
- Replace the PALS with GALS.
- Upgrade the SCSI Chip to the Proto 08 or AMD equilvalent.
- If you have a battery on the motherboard, remove it yesterday. Go with a coin type battery or wire it off the motherboard. I have seem far, far to many 3000 die from battery leaks.
- A buster Rev 11 is necessary especially if you want to use z3 cards like you mentioned.
In addition here is a 3000 Guide that covers your questions:
http://www.amibay.com/showthread.php?59870-Amiga-3000-buyers-guide-myth-buster-and-Refit&highlight=mythbuster
