From quickly reading through this thread I think you've already got all of the basics covered: you have most of the engine, suspension, transmission and appearance modifications you can get within your criteria already on your car.
That means it's time to take care of the details that will add huge amounts of cool factor to your car. These were all factory options in Europe only, which makes them that much cooler:
- Full Amaretta and cloth vader interior - take it to the next level
- Powered rear vent windows - these are a must as nobody has them
- Red BMW Motorsport seatbelts - rare as hell and very cool
- Underhood lamp - nice detail that comes in handy
- Footwell lighting - a surprise party for your feet
- Rear foglights - to easily confuse people
- Headlight aim control - blind your opponents
- Rear sunshade - for those sunny days
- BMW fire extinguisher - when things reall heat up
- BMW emergency kit - in case she bites
If you need any other suggestions, let me know.