I was in the same boat as you at the end of last year. I ended up buying a fully done up E46 M3, and I must say it destroys the E92. I can't say it's cheaper than the E92 other than the tires seem to last longer, but the pads for the BBK are very expensive and so are the rotors. Then I also had to get a trailer since the car doesn't have cats, sound deadening, or even a stereo. Oh and the heavy springs and stiff suspension would destroy my back and bend my rims if I hit something. I thought that I would just save money by buying an already heavily modded car rather than a mostly stock one and doing my own mods, and in the end that might be true. However, in this case I have a non-streetable car, so you might want to get a fairly stock E36 or E46 so that you can still drive it to the track and have fun. I met two guys from NYC who have well street/track modified E46 M3's and they are stupid fast at the track. So, in the end remember that the best thing you can do to lower your lap times isn't a BBK or coilovers, it's seat time at the track with good instruction.
"The laws of physics cannot be repealed, even with DSC." - BMW Owner's Manual