Engine work on the E30 M3 gives new meaning to the word expensive.
Needless to say, I haven't done much. Here is what I have done.

EVO Air Box with K&N Filter

A fairly inexpensive and simple upgrade. You can order these parts from Turner Motorsport. The EVO Air Box has a different intake and supposedly flows better. It replaces the bottom piece of the Airbox and has a new intake pipe. To install you need to remove the front bumper so you can put in the new intake pipe. The rest you can do from the engine compartment.

Timing Chain Replacement

Ok, here it goes. My engine has 140k on it with the original Timing Chain parts. I figured it would good insurance to replace these until I can save up enough to do a 2.5l upgrade (which by the way will move me to I-Stock.) Since you have to practically take off the whole front of the engine, I figured I would change all the Timing Chain parts in there. This is quite expensive, the parts alone cost $700. The expensive pieces are these tensioner rails which the chain rides against. For the life of me, I do not know why they are so much. They are just metal rails with a hard rubber on them. Anyways, it will be another $600 in labor to install the stuff.

Oil Pan Baffle

A must for tracking or racing any E30 M3. This little piece of aluminum bolts inside your oil pan and helps keep oil around the oil pan pickup under hard cornering. Cheap and inexpensive insurance for your engine. You can get these from Turner Motorsport or Korman.

>New Belts

I replaced all the fan/accessory belts. Nothing like breaking or throwing a belt to end your day. Again very inexpensive insurance. Three belts ( Alternator, Power Steering, and Air Conditioning.) I will take the A/C belt off while the car is on the track. This is allowed under J-Stock rules.

New Hoses

You can buy an entire Cooling System hose set for about $115. It is probably a pretty good idea to replace all of these before you start subjecting your car to the extreme heat of racing.

New Bushings in Engine Compartment

Just about everything made of rubber on my E30 M3 was shot. I replaced all the bushings in the engine compartment. I used Turner's Urethane Alternator Bushings, and I used the stock factory ones for the Power Steering Pump and the A/C compressor.

Here are the parts I used.