The drop with a car that is on non sport stock suspension is likely going to be in the 2-2.5in range. The Eiback Pro Kit (which has one of the more conservative drops on the market) is 1.4 in front and rear drop on a sport suspension car. My friend has this on his 325Ci Vert and it looks great but it also needs spacers since he is running 135M style OEM wheels with stock offsets (+47f and +50r).
As far as what wheels to get, depends on your budget. If you budget is relatively tight, Velocity Motoring CSL reps or V710s are a good choice. Available for about $1000 (not including tires) new in 18 x 8.5in with a ET35 offset. These look great (as evidenced by how many cars are running them). Also, it is an OEM look. Used OEM rims are usually pretty plentiful around the forums. The aforementioned 135M is the rim that comes OEM with the ZHP package. 4 OEM M3 fronts will fit on your car (18 or 19in). BBS wheels are nice but $$$. LMs and CHs are common choices on E46s as well as RS-GTs, REs and RGRs. The last 3 are very expensive due to the fact that they are forged and they only come in 19in sizes. BBS are very high quality though.
Just some suggestions anyways.
2009 Interlagos Blue on Black Novillo M3 Coupe DCT, EDC, MDrive
Volk Racing LE37T 19 x 8.5f, 19 x 10.5r, Toyo Proxes T1R 24535R19f, 275 30R19r, OEM CF ZCP Splitters and Rear Spoiler, GC Coilover Conversion 440f, 550r