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, 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, Advan TC3 18x9.5 & 18x10.5 - Hankook RS3 (Track), HRE R40 - 18x10 & 18x11 - Michelin PSS (Street)