ye like potter already said it will cause balance issues.
Also i think choosing a car should just depend on the cars look and its handling and not the special weapon it carries.
A possible alternative would be having just a special power up chosen undependent of the car. Or just give the option to upgrade a power up already in the game instead of having a complete new one.
But then again there is the balance issue, to make all setups worth using. However, with just an upgrade like faster turbo, harder hitting missle or higher chance to get the assassin, it could be easier to balance things out.
There are more ways to make the weapon system more complex (some discussion in other threads, e.g. this
), but some times a more simple version is enough.