Suspension & Chassis
Suspension Techniques Anti-Sway Bars
These anti-roll bars are one of very few modifications that are worthwhile on a street-driven car. They make a huge improvement in handling, for a relatively low price. Keep in mind that you will need to reinforce all of the rear mounting points. They will tear out, and that is a huge hassle to deal with. I gove these a big thumbs-up though!
Ground Control Rear Shock Mounts
I prefer these mounts because they are far more durable than the stock E30 ones. They do not seem to transmit excessive noise or vibration into the car, so I generally see them as worthwhile. They have polyurethane bushing inserts that can be replaced for about $15.
Zimmerman Crossdrilled Rotors & Axxis Ultimate Brake Pads
So far, the Axxis Ultimate pads are the best that I have used. The cold bite is great, and they are amazing once they warm up. Dust is relatively low with them, and squeaking is very uncommon. The set of Zimmerman cross-drilled rotors don't really yield any improvement in braking, but they have not warped at all (which is more than I can say for a few others I have tried). When these rotors wear out, I will likely use some OEM Brembo blank rotors.
Stainless Steel & Teflon Coated Brake Lines
For about $100, this is one of the more worthwhile brake system upgrades. This set was purchased from Bimmerworld (on sale) and were definitely worth the money. Be sure to bleed your brake system VERY well once you have installed these. I would recommend capping all open lines during installation. If air gets into your ABS unit, it is very difficult to get out yourself.
Bilstein HD Dampers
I use these in place of the stock dampers. They provide a slightly stiffer ride, with a noticeable improvement in corner stability. They are pricey at $430 per set, but I consider them to be worth the price.