A cold garage is basically unusable for half the year, but adding a smart electric heater changes that entirely. You get a space that stays comfortable for projects or parking without wasting energy when you are not out there. Getting it wired and mounted usually takes an electrician just a single morning, so you will not have workers camped out in your house for days.
Early autumn is the absolute best time to book this work before heating season hits and contractors get buried in emergency calls. You want to reach out to at least three local pros for quotes so you can spot any outliers and negotiate a fair rate for the install.
Keep in mind that the 0% intro APR is a hard deadline, not free money. If you still have a remaining balance when those 15 months run out, deferred interest will kick in on what is left.