A good suspension lift kit changes how your truck handles trails and looks in the driveway, but it is not a cheap upgrade. Between the kit, new hardware, and alignment, you are looking at roughly $2500. Shop labor rates usually dip in the slower winter months between January and March, making that the smartest time to schedule the work.
You want to separate the shop negotiation from how you pay for the bill. Get your out-the-door price locked in writing first, then figure out your payment strategy for the deposit and parts.
Keep in mind that 0% interest is a deadline, not free money. If you still carry a balance when the 18 months run out, standard interest kicks in on the remaining amount.