I think if you have the money to spare, buy everything you can to gain an advantage. Its not like the player is saving up for a new car as soon as they get the 8L. The side impact shields will save money on car damage, and everything else seems to help. I never buy missiles, but I guess knocking out the 2nd fastest car will often help you catch up to the leader.
liviux76 - Thanks so much for the spreadsheet! Yes I can suggest a few changes, and I'll use my notes to produce a slightly more accurate version as a help file for the Lemon database Super Cars page. That way, anyone can find these when searching for the game. You definitely saved me typing them up.
Harry - The battle for 4th continues!