A clean engine bay is the difference between a car that looks neglected and one that gets noticed at a show or sells for top dollar. It is a specific type of maintenance that requires care, as you do not want to flood sensitive electronics with high pressure water.
Detailers are often slowest in the dead of winter or mid-summer, so ask for a quote then if you want to avoid peak season pricing. Getting this done properly costs about $300, which is manageable if you plan it against your regular car care budget.
The 0% intro APR is a deadline, not a gift. If you have not paid off the full $300 by the end of the 15 months, the bank will charge you interest on the remaining balance at their standard rate, which will cost you more than the detail itself.