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.
'13 SSII M3 Coupe - KW Clubsport 3-way (Eibach Front Spring Conversion), Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Mode Carbon GT4 Splitter, Advan TC3 (18x9.5/10.5), BBS E05 (18x10/11), G-Force Rivals (275/35/18, 295/35/18)