When I had my '89 325i (it got totaled on New Years day 2004) it had the following mods:
H&R OE Sport springs, Bilstein HD shocks & struts, Offset control arm bushings. E36/46 cabrio rear shock mounts, Eibach frt stress bar, Eibach sway bars, Conforti/Turner Motorsport computer chip, New E30 M3 non-airbag steering wheel, redone fabric interior, Borbet wheels, keyless entrysystem and other lesser items.
Sylvania Silverstars headlight bulbs are a definite improvement if you do not want to go to the Euro system.
One thing you may want to consider for convenience is a second trunk light. When you open the trunk on an E30 the light becomes almost useless. A second light can be added by installing one in the fibreboard behind the rear seat and tapping into the existing light for power. A new light from the dealer costs about $3 plus some wire. I wrote a short article on doing it for the club newsletter if you are interested.
Another item concerns the front lower grille openings. The lower grille openings only have one plastic insert while the other is empty. You can add a second insert ($20) into the opening but it requires making up a few brackets from 22 gauge sheet metal you can get at Home Depot (about $5). Good qualty structural epoxy is also needed. It works nicely and I think completes the front end if you are not buying a new front air dam etc. I also have an article written on this mod.
The 1989 I had is the 2003 class winner in the Bavarian AutoSport photo contest and can be seen on their website as the car was before I did the grille change.