You'll probably want to head over to R3VLimited.com, Bimmerforums.com, or a site similar to those - they deal with those cars moreso than we do, since this site is dedicated to the 318is.
Having said that:
The M-Tech kits are nice, and are factory, while Rieger are aftermarket. I have no experience with Rieger, but I do know I wouldn't put a roof spoiler on anything I drive that didn't need it to keep it from flipping over at high speed. Which would be no car....

No bad info on Abraham Motorsports, and I think some people have recommended them.
No idea on power for the M20 - it's not an M42.

Springs, all I can say is I would not recommend Eibach, which are what I am running. Stance and drop is good, but the spring rate feels essentially unchanged from stock, which, on a lowered car, is not such a good thing. I've not had any problems with mine, but I drive extra carefully! H&R have a good rep.
Strut bar, I'm a fan of the Sparco bar. Solid, inexpensive, fits well (except on an M42-equipped car, where it comes *really* close to the intake manifold).
Side exit straight pipes? Like, no muffler? Building a track car?
HIDs... well, let's just say I don't like them on cars that didn't come with them, particularly since most people who install them don't give a rat's ass about properly aiming them. Do yourself and everyone else a favor, and if you put them on, aim them properly. Blinding everyone else on the road so you can see is not going to win you popularity points.
Most folks use 3M or Lamin-X film to go yellow. There are yellow headlights and foglights available from BMW, but they aren't cheap and the film works to protect your headlamp from chips anyway.