Keeping a car past 100,000 miles saves money until the suspension goes soft and you start feeling every pebble in the road. Running an older vehicle means planning for wear items that suddenly hit all at once. If you need major maintenance, January and February are usually the slowest months for mechanics, making shops more open to price adjustments to keep their bays full.
Always negotiate the job before you figure out how to cover it. The financing plan is just a tool to spread out the cost of parts and labor so an unexpected repair does not crush your monthly budget.
Keep in mind that 18 months is a strict deadline, not free money. If you still carry a balance when the promotional period ends, regular interest charges kick in on whatever remains, which defeats the purpose of spreading out the payments.