im not going to make this a long one, cause i know ppl rather read a short one..
I would say you should do like this:
If you wanna get all the good items fast, buy stuff in this order:
1. lvl 1 wheels
2. First Rocket
3. lvl 2 wheels
4. ''Handstand'' Trick
5. second rocket
6. lvl 3 wheels
7. third rocket
After this, buy all the groupie's
But be carefull.. If you die, when you have other ppl then you in the car, you have to buy them again.. Thats why you shouldnt buy them before you got all the other items..
(notice) Groupie's make you get more points