A solid mechanical rower is a lifetime buy. You want air or water resistance with standard chain hardware that won't snap in three years, so buy once and skip the flimsy under-$300 knockoffs.
January is the sweet spot for gym gear as brands discount older stock to make room for new models. Grabbing last year's build gets you the exact same flywheel and frame for hundreds less.
Keep in mind that a 0% period is a strict deadline, not free money. If you still have a balance when those 18 months end, you will get hit with the standard variable interest rate on whatever is left over.