I thought this was about mechanical upgrades?
Turbo... or simply buy an e34 with the next largest engine
There really isn't much you can do without spending a load of cash. The car came from the factory in a high state of tune and most bolt on mods don't offer much actual gain. I made mine loud and it makes me feel good and, the chip did help. Beyond that I wouldn't do much. Just keep the maint up.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5