It;s not Bond, so you cannot modify the car THAT much, it doesn't fit tyhe style...
A tank would be overdoing it- the system would notice. Sometimes it's best to play to mood rather than logic with these things.
But as for big guns- well, the OTHER team had such a weapon in their vehicle...
Mind you, I'll make limits, you cannot bring artillery pieces or endless RPG launchers or whatever. I would probably let you bring ONE such big gun.