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 M3 - Silverstone II - Sold
'04 M3 - Jet Black - BBS E88, KW Clubsports