First things first, if you have any desire to modify an E30 or drive it aggressively, please, for the sake of your own safety and that of others, make sure the car is in good mechanical condition right off the bat. It doesnt have to be perfect, and definitely not pretty, but make sure the major components WORK. Brakes, steering, clutch, tires, etc should all be in usable condition. Wear items under the hood (vacuum hoses, paper seals, etc) should be inspected. If you are not willing to take the time to do this stuff, or alternatively pay someone else to do it, I strongly suggest you re-assess your decision to have an E30. They are good cars, but certainly not for everyone. The newest E30 is 13 years old, and as such, stuff goes wrong, stuff that needs to be fixed. Now with that out of the way, Ill get to the more fun stuff.
For me personally, a car is a compromise. It should be fun, but it should also be practical and reliable. Keeping that in mind is important when choosing what you want to do with a car, modification wise. Just as important is deciding exactly what you want the car to do for you. Fun, quick commuter car? Balls out gutted track rat? Somewhere in between? Whenever making choices, keep your goals in mind. Its very easy to get caught up in the rat race of buying and adding parts, just to buy and add parts. Any time you change part of the cars mechanical equation, you should be aware of the consequences.
The first thing I suggest doing for an e30 owner (assuming you have a 5 speed manual transmission) is a clutch stop and short shift kit. You can do both with ~$60 invested, in about an hour, with basic hand tools. Using an OEM BMW shift lever, a few new bushings, a few nuts and bolts, and the clutch and shifter will feel much shorter and quicker. You use these controls every single time you drive your car, so youll notice the difference right off the bat. These are little mods, but IMHO two of the most rewarding in so far as driving fun. They add no horsepower, and arguably dont make the car any faster, but they do aid in the fun factor.
Now were getting into bigger territory. More money, more involved work, more possibilities to screw up. However, if your suspension is in good shape as is, meaning the ball joints and bushing and such are in good condition, adding a set of aftermarket struts/shocks and springs can make a world of difference in the handling of an E30. BMWs for the most part are good handling cars. They are well balanced and have great steering. Adding a little stiffness and lowering the center of gravity a bit can make good handling E30s handle wonderfully. However, you can go too far with suspension. Too low and too stiff, and youll find yourself scraping all the time, or breaking oil pans, or worse. So keep in mind if you plan to drive your car on the street, you dont want suspension that is too aggressive. There is a reason the H&R Sport spring and Bilstien Sport shock combo is so popular. It handles well and looks good, but is still streetable. Whatever you decide to go with, Id advise you to get an alignment after the install to maximize tire life and handling.
Power: This is probably the most talked about subject amongst the E30 community. However, Ill tell you this, take it for what its worth. About 90% of the crap you read about that says it gives you power, doesnt. It might make noise, or feel faster, but usually, it doesnt work as advertised. Another 5% of stuff does work, but costs a helluva lot (strokers, turbos, etc), and thats the subject of a whole nother thread. In all actuality, a tune-up will offer just as much performance gains as many so called power adders. But thats not to say they dont have their place. If you are like me, you enjoy the sounds of cars. An intake and exhaust system, while maybe not adding much HP (if any), can definitely make a car more enjoyable to drive and listen to. ECU programs chips are one of the few mods that actually do something. With a quality chip, an E30 can gain a little horsepower and torque, increase its redline, and rev a little quicker. They are a subtle gain, but help nonetheless. As they sit, most E30s are quick enough to have fun with, so focusing resources elsewhere is a smart idea.
Often overlooked unfortunately, but brakes are a critical component of driving. A good set of pads, quality fluid, and maybe some stainless steel braided lines can go a long way to lowering stopping distances, fighting brake fade, and increasing confidence.
Performance Info courtesy of r3vlimited.com