If you are looking for the best modifications you can do to your BMW 3 series from 1991 to 1997, here's my top 3 list for the modifications that you will notice the most improvement.
Although a set of top of the line Bilstein or Koni struts and matching springs will make a difference, if your suspension is worn out then you will probably notice the most improvement by replacing any worn ball joints and bushes. New front control arms, bushes, track rods, rear shock mounts, rear trailing arm bushes and sway bar links are the usual culprits for poor handling and noisy or clunking suspension.
I recommend finding a BMW specialist mechanic who will put your car up on a hoist and tell you what needs replacing.
2. Sports seats
There's no point making your car go faster around corners if you are slipping and sliding on factory seats.
The factory option sports seats are good enough for enthusiastic street driving and are was to find if you have a 2 door. Factory sports seats for sedans will be difficult to find.
Aftermarket racing type seats will als be good for cornering but possibly not so good for comfort and potential legal and insurance issues.
1. Steering wheel
You may be surprised but my number one modification was fitting a modified factory steering wheel with a reduced diameter and thicker rim.
You don't realise how big and unsporty the standard wheels are until you try a wheel like this.
You could go aftermarket but might have issues with legality and safety.
Mine was modified by a company in Germany
and looks factory and retains the original airbag. At about A$500 it's a big expense but it improves the driving experience more than anything else I have done.Footnote.
Why is there no performance modifications that actually make the car go faster on my list, you ask?
Although engine related modifications such as exhausts, air filters, chips, etc can be fun generally they don't provide much if any noticeable difference, except maybe noise. If you want a noticeable difference then you are best going forced induction if you are set on modifying your current car. Otherwise you will probably spend less money buying a more powerful model, eg. Going from 318 to 325, or 325 to M3.