A used Silverado is a tool that happens to have seats. It handles heavy loads without complaining, but it will cost you at the pump. Expect to spend a few hundred a month just on gas if you are driving it daily. We see the best prices in late summer or early fall when dealers are desperate to clear the lot for the incoming year's models.
Negotiating the price is a separate game from how you pay. Get the price down first. Then, use a specific interest-free window to cover your down payment or the whole truck without the bank taking a cut.
The 0% interest period is a hard deadline, not a gift. If you still have a balance after the 18 months, the standard variable interest rate will apply to the remaining amount and the debt will become much more expensive.