Cars are essentially giant depreciating assets that demand constant attention. When your alternator fails, your battery stops charging and the car will eventually refuse to start, making this a non-negotiable expense.
Independent shops are almost always cheaper than dealerships. Because this is an emergency repair, you likely won't find a seasonal discount, so focus on getting the labor cost down instead.
The 0% period is a deadline, not a gift. If you still have a balance on the card once the 18 months are up, the bank will start charging interest on the remaining amount.