I think one of the best mods is installing the M3 rear subframe bushings. It not only firms up the rear end but also seems to make the front suspension work better. This will get rid of that rubber band effect when you turn the car hard into a corner. No more jiggle or wander from the rear end under power or cornering G's.
Have you thought about BMW's Performance Suspension Kit? For the 135i it comes with shocks, springs, and a front sway bar(I think?) plus all the mounting hardware. I think its priced around $1,100 for the kit.
Lots of guys install the M3 front sway bar. But I think that can only increase understeer at the limit. The reason why everyone seems to like this mod is bc they get no where near the limit on the street - and the stiffer bar really cuts down on front end sway and makes the steering feel more direct.
One of the Best mods for the suspension is getting rid of the RFT and going with a top tier tire like Michelin PS3's or PSS's. That was one of my best mods to date along with the M3 rear axle bushings.
Dackel