What is your budget?
You may be able to get some used HRE's or another option is to run (4) 19x10 OEM GTS rear wheels (black). I have seen it done and it looks fantastic. You accomplish the concave look, price is reasonable, and you avoid 20" wheels which sacrifice performance. I don't think you gain much "concave" by going from 19" to 20". You lose performance, they cost more, fewer tire options, and more susceptible to damage.
KW Clubsport 3-way, Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Bumper & Black Mirror Covers, Turner Motorsports Studs/Lugs & Delrin Diff Bushings, MRF Solid Sub-Frame Bushings, RD Sport Sways, Recaro Profi-XL & Planted Seat Brackets, BMW Performance Carbon Fiber Interior Trim, BMW Edition Grills/Gills, Mode Carbon GT4 Splitter & GTS Wing, Advan TC3 (18x9.5/10.5), BBS E05 (18x10/11), G-Force Rivals (275/295)