At this stage in your life, I'd recommend staying away from revolving credit (credit cards) and car loans.

It sounds like you have enough cash in hand to get a good used car, and enough sense to maintain it well. Do that, and put a couple of hundred dollars a month into a savings account instead. What you're doing is saving what you would have been paying in car payments. By the time your 'good used car' runs out of useful life, you should have enough in the bank to pay cash for a good used MB and skip the car payment.

I've never found sitting on a credit card balance to help your credit score. I've always paid off my cards every month (and I've never had more than two at a time), and have a very high credit score.