A solid commuter scooter should get you through at least two years of daily use before the battery starts to fade. Focus on build quality and folding mechanisms so you aren't stuck replacing a cheap frame or hinge after six months of city transit.
Scooter brands usually refresh their lineups in the spring, which means late summer is the perfect time to find last year's model on sale. You get the same reliable tech without paying the premium for the newest paint job.
That 18-month window is a deadline, not a gift. If you still have a balance when the period ends, the bank will start charging interest on whatever is left, which defeats the purpose of the plan.