October is the best month to buy a backyard wooden playset because stores want to clear out heavy outdoor inventory before winter hits. Once it is built, your weekends change from figuring out how to entertain the kids to watching them climb and swing in the yard while you drink coffee on the patio. Putting the kit together takes a full weekend and at least one extra set of hands, so plan to rope in a neighbor for Saturday morning.
Before you hand over any money, get quotes from at least two local handypersons if you decide not to build it yourself. Prices for assembly vary wildly, and comparing two bids keeps everyone honest. Once you have a firm total for the wood and the labor, you can look at the real numbers and map out how to pay for it without blowing your monthly cash flow.
The 0% window is a deadline, not free money, and any balance left when those 18 months are up will suddenly start charging regular interest.