You want an enclosed pro-grade 3D printer so you can stop babysitting failed prints and start building parts that actually fit together. Buy a solid frame and decent filtration the first time out, and you won't be shopping for an upgrade by next winter.
Watch the release cycles closely around summer maker events. When the manufacturers drop new chassis revisions, last year's flagship model usually dips in price while still keeping all the parts availability you need.
Remember that the introductory window is a strict deadline, not free money. If you still have a balance left when those 15 months run out, the regular interest rate kicks in on whatever you owe.