A rolling diameter size calculator I use is one from
1010tires.com. Very helpful and easy to use!
Aaron, give Peter @ OZWheels a call on 0404844806. His shop is in St Marys (previously Leppington) and I remember he makes Porsche replica wheels, except in multi-piece form. They're custom made to your dimensions/specs/colour/finish, but don't come cheap at $650-ish per wheel before tyres. Check out
http://www.ozwheelaustralia.com.au/2.html - it hasn't been updated for years though and last I went there was more than a year ago, but I think he should still make them.
http://www.vwwatercooled.com.au/forums/