The 2004/2005 BMW wheel catalog shows the 20" wheel as a Star spoke 87 with the part number 36 11 6 753 516 for the 9.5x20 front wheel and part number 36 11 6 753 517 for the 10.5x20 rear wheel. MSRP price for the front is $695 and $735 for the rear.
Each wheel requires the BMW Hub cap part number 36 13 1 095 361 MSRP is $3.75.
Note that the catalog indicates that the wheel can only be used with the flared wheel arch or sports accessory kit including wheel cover.
My suggestion is that you check with your nearest BMW parts counter representative.
A forum community dedicated to BMW owners and enthusiasts. Come join the discussion about Bimmerfest events, production numbers, programming, performance, modifications, classifieds, troubleshooting, maintenance, and more! Bringing the BMW community together.