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.
'04 E46 M3 - Jet Black - Sold
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'13 E92 M3 - Alpine White - 1 Option (DCT), Ohlins TTX, AP Racing, Cobra Seats, Akrapovic GT4, BBS E88, Volk TE37, M3 GTS - Steering Wheel, Aero, 1/2 Cage, Seat Delete