A lot of car dealers will add a surcharge to the price of the car if you pay by card to cover the charge they will incur from the card agency. Most shops already increase their prices because of this (strangely you dont get a discount if you pay cash..lol) but car dealers dont as most people will pay by cheque/bankers draft or direct transfer (or loan). I'd recommend you speak to some of the car dealers you are looking at using.
As for it affecting your credit rating, if you are paying off regularly and on time, it can actually do your credit rating good. My dad had only had a mortgage and consequently when he went to get a credit card it took him ages to find a company willing to do it as they all said he wasnt viable due to not having any credit history...hehe
The downside to the credit card is you pay a load of it off, then suddenly the looks of a nice shiny new TV pops in to view and you think...well, the plastics there for a reason I guess...and bingo....your hooked, they make money and your risking your financial security.
If you know you can pay off the card then cancel it, go for it. If you think you might be tempted to keep using it, get a loan