yeah, find that out cause I would like to know as well...
If you want maximum performance, Autotechnica is the man to talk to with a 318...suspension is the best thing to work on for perfomance aside from helping the cars breathing. 318's are incredibly gifted with handling because they are much lighter than the 6cyl 3's.
However, if performance is what matters, think about the 4 door 318i. It has been popular belief that it is actually lighter than the 2 door IS model. The IS supposedly has a reinforced frame because not having all the same pillars as the 4-door frame causes it to be less rigid and less stable, giving the reason for the reinforcement. This reinforcement adds extra weight.
Also, you might want to look for a 318 that is a 95 model or older so you can get the OBD-I computer system, which is easier to tune, allowing for better performance when upgrading parts...
However, if you are a patient person, you will be able to find a 325 for similar prices as a 318, its just a matter of searching and the performance increase and power difference is worth it. High mileage isn't a huge deal so long as the car is taken care of. These cars are bulletproof if you take care of them...many people on this site have Bimmers that have last 20+ years with over 200k miles on the clock, and still running strong as ever...
Thats all just some insight, but in answering your first question...there is plenty of stuff out there for the 318, but the gains aren't all that significant until you start putting serious money into it (FI, engine internal work, engine management software swapping, upgrading, etc)
5 Speed Manual
Alpine CDA-7895 Head Unit
12" Alpine Type-R, sealed box
Pioneer 400w amp for $20 at a yard sale (to replace my fried MRV-1507)
Infinity Kappa 4" 2-way speakers(rear)
Infinity Kappa 5.25" component speakers(front)
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