Hey, nice to see another person that likes e28s! Well, let me first say that I have a 1984 533i. I looked for parts everywhere. A lot of my specialty parts that I get though are off of eBay. There's a lot of cool stuff on there for e28s. Also, your prob familiar with it but if you not...Bavarian Autosport has a lot of replacement parts and maintenance stuff. The only problem is that they can sometimes be expensive.
As far as motor swaps go, an easy one to do is the M30. It’s pretty much a direct bolt in. As another option, you can take the turbo engine from an e23 Euro 745i turbo and that is a good bolt in. Or you can just do the M30 and take the turbo, manifold, plumbing, etc from the 745i and put it on the M30. I have not done it but from what I have heard….M30s LOVE turbos…and are no slouch when it comes to performance. You could do a newer engine but it will be more complicated and expensive.
As far as exterior mods go, I’m not sure where you could pick up some e28 M style aerodynamics. Maybe eBay or looking around at some of the bmw forums under the classifieds. When I went to work on the visual appearance of my 533, I had wanted to make it look like the e28 M5. Body parts form e28 M5s are not uncommon, but are usually expensive…unless they are crap. I bought a nice front spoiler off of ebay. Very aggressive looking….especially from the front. I think its made by Zender. When your driving and I come flying up behind you it looks intimidating. Especially since my car is lowered. For the sides, I didn’t get any side skirts; I just painted the bottom to match the body color. And for the back, I managed to find a rear apron by zender. Then I added some Mille Miglia MMII-2 Sport rims and tinted the windows……def looks bad ass.
For lighting, I have Hella head lamps with PIAA Extreme White bulbs….very bright. And for the rear I kept the stock e28 lights but I smoked them so it looks very nice. I also custom made some side lenses out of plexi glass. I kind of traced the outline if that what you want to call it then cut them out and fitted them. Then I put a light smoke tint on them to complete the look.
These are just some of my ideas and some examples of what I did. There are many more options. Hope this helps.
Tony