Seems like a nice idea, provided the caps are quite low. We don't really want jeeps exploding with the force of a mini-nuke.
Based on an already submitted minigamepoints x 5 = Modification, I think a vehicle should begin with 0% "upgrades" and as you add more modifications, it goes up 10% at a time until the vehicle is twice as fast, twice the explosion etc. This way, the cap would be at 200% of the vehicle's original capability.
OR
Same as above, but every time you add a modification, the cost for another mod becomes higher. Eg:
Player Modifies Speed for 5 Points up to 10%
Player Modifies Explosion for 5 points up to 10%
A WHILE LATER
Player Modifies Speed for 10 points up to 20%
So it would go:
Upg 1: 5 pts
Upg 2: 10 pts
Upg 3: 15 pts
Upg 4: 20 pts
Upg 5: 25 pts
Upg 6: 30 pts
Upg 7: 35 pts
Upg 8: 40 pts
Upg 9: 45 pts
Upg 10: 50 pts
To max the car's stats on one type it would cost 275 points.
I know this may not be possible with scripting for a lot of people :/