Imagine walking into the backyard and picking fresh tomatoes without bending over or dragging a heavy hose. A raised bed system with drip irrigation does the heavy lifting for you, and it usually takes a weekend or two to set up. You want this finished before spring hits so your plants have a full growing season.
Buying lumber and soil in late winter is the move because prices often spike once everyone starts gardening in April. Get at least three quotes from local landscape supply yards for the bulk soil and mulch. Delivery fees vary, and buying in bulk is always cheaper than dragging forty bags from a big-box store.
This plan only works if you pay off the full balance before the 15 months are up. If you still owe money after that window, the standard interest rate kicks in and the garden becomes a lot more expensive.