the best thing you can do for that car would be a Conforti chip from Turner Motorsports (www.turnermotorsports.com
). exhaust wise, try to find a complete exhaust, from the manifold to the cat, out of a stock e30 87-91 325i/is/ic, then look for a decent stainless steel cat-back system; either custom or a cheap bolt on using the stock 325i diameter dual pipes.
no matter what you decide to do, you should always keep in mind that the 325e was bmw's economy car, and the engine's head design was heavily restricted in favor of fuel economy. therefore, nothing short of a stroker rebuild or engine swap will make the car fast.