That clunk over every bump means your suspension is shot, and once you pass high mileage, a full strut and shock upgrade is the only way to get your steering and ride quality back. Parts and labor for all four corners usually land right around $1500, which is a lot to swallow all at once.
Shops are slowest in mid-winter and right after the summer road-trip rush, making those the best times to get quotes without paying peak labor rates. Negotiation happens before you talk about how you are paying, meaning you pin down the exact part numbers and labor hours first, then figure out the payment method.
Remember that the 0% intro window is a strict deadline, not free money. If you still have a balance left when those 18 months are up, standard interest kicks in on the whole original amount, which will wipe out any savings you gained from timing the repair right.