1. you were following too closely if you couldnt stop in time to not hit anything. bmw has better brakes than any van - guaranteed.
2. carbon fiber hood, new bumpers, splitters and what not will cost you more than the car itself.
3. engine swap = costs too much. if you are looking for better acceleration look into a different differential
4. don't go with nitrous. in the long run forced induction is much better for the engine/more reliable than nitrous ever will be. a turbo kit will cost you a few thousand... and it's always there - constant performance. nitrous = you spray twice and you are back to being slow until you refill it, which costs an arm and a leg.
anyhow, welcome to the forums.