Welcome! Not sure if you have a six-speed or not, but if you do then I would highly recommend a lightweight flywheel. This modification really awakens the car. You could also get a 3.15 differential from an automatic 540 which would provide a noticeable improvement.
In my experience with my own 540, unless you plan on spending really big money like a VF supercharger, then the above mentioned modifications are two of the most noticeable.
Angel Eye Kit is a good question. I was fortunate enough to have the Hella replacements done by the previous owner. Since your car is relatively old like mine, you should consider a cheaper alternative from Ebay like the ones in this link:
http://search.ebay.com/E39-Projector...opZ3QQsbrsrtZd
You will need to replace the entire headlight assembly as your vehicle came equipped with non-projector headlights. When you upgrade to the projectors they typically come equipped with angel eyes as well.