I just ordered the GT3 kit from Flosmann for 13k (that's including vat and shipping). I opted not to get the wing or the exhaust chimney to save some cash. I figure there are plenty of wings available here in the states for less than 2k euros.
Obviously its not OE BMW but Flosmann has an excellent reputation for making remarkable parts. This kit is Carbon Fiber with epoxy resin.
http://www.racingparts-bmw.de/start/...php?wog=z4.htm