A solid monitor arm is about ergonomics and sanity. If you buy a cheap plastic version, it will sag within a year. We want heavy-duty steel or gas-spring arms that handle at least 20 pounds per side. These should last as long as your desk does.
You do not need the newest model with built-in lights or USB ports. Last year's basic versions from reputable brands work just as well. Timing this for August back-to-school sales is the move for the best prices on office gear.
This 0% interest period is a hard deadline. If you still have a balance after 18 months, the standard interest rate will apply to whatever is left, so make sure the desk is cleared by then.