You will get the lowest common denominator prize car. If you want the silver car, you'll need to get at leasts all silvers excluding coffee breaks for a particular license. Your results for a different license have no bearing on the one you are currently doing. For example, if you get 15 golds in iB, then 1 gold in iA, you won't get the iB gold prize.
Second question, if you get all golds you get the prize cars from bronze silver and gold. If you are currently all bronze you can upgrade to all silver/gold and get the appropriate prize cars.