The '94 318 is a good, solid reliable car that still has years of good running left in it if looked after. However, it was never designed to be cool.
If you want to spend a fortune on changing a car, this is not the one to do it to. It is possible to spend lots of money on AC parts etc. but this will not change the fact it is the 1.8L engine so will add nothing to resale value. It is also not the kind of car to keep as a classic, it never will be.
However, it is simple and cost effective to give it a makeover that will make it look a lot better and more attractive to this type of cars market when you decide to upgrade to something else.
1 Badgework. Take the 318 badge off. Won't make it go faster but will not advertise the smaller engine size. Use a hairdryer to soften the glue and then dental floss to remove the badge without scratching the paint. Leave the boot blank and DO NOT fit M badges. It is simple to tell an M3 from interiors etc so zero cred if you try to fake it.
2 Exhaust pipes, the higher power 3 series such as the 2.5L were fitted with chrome twin tail pipes. Buy a cheap aftermarket screw on pipe cover and fit that. After 1 & 2 car will then be difficult to distingush from the higher power models.
3 Clean it and keep it clean. Use good quality polishes. Look at the big name ranges and buy the most expensive they have. They are easier to use and produce a better result. The low end stuff is crap and produces a poor result. Use body polish, glass polish, alloy wheel polish. The best AC wheels in the world look totally crap in they are covered in brake dust. A polished car repels dirt so once a good layer is built up, it is easy to maintain.
4 Body parts. If you want to spend a bit more look at M style mirrors, they do look good and have better visibility. BMW are expensive, look at third party. Another option is a rear spoiler. Do not go for a coffee table. Look for a low subtle spoiler that is more in keeping with the 4 door body. Again, BMW are expensive, look at third parties or e-bay. Key to this is subtle, OTT will not work.
5 Wheels, suspension, engine chips etc. You will run up big bills here with little gained. Save the money until you have your M3, M5, 540 or 8 series. Money spent on these models will give you much more enjoyment.
Hope this helps and is taken as constructive