Slapping a set of custom forged lightweight wheels on your car completely changes how it handles and how it looks sitting in the driveway. Those unsprung weight savings mean sharper steering and better grip, but you are looking at around $2500 just for the set before you even think about rubber and mounting. To keep the cost down, you want to shop for clearance sales in the dead of winter when demand drops and shops are trying to clear out old stock.
Never mix up negotiating the price with figuring out how to pay for it. Hammer out the best deal on the hardware first, and only look at financing once the final price is locked in. A payment plan is just a tool to spread out the cash hit, not a discount on the price itself.
Keep in mind that the intro window is a hard deadline, not free money. If you still carry a balance when those 18 months run out, the regular interest rate kicks in on whatever is left.