A quality espresso machine with a built-in grinder is an investment in your morning routine. If you choose a reliable model, you should expect it to last for years rather than replacing cheap equipment every few months.
Wait for the late November holiday sales to catch a price drop. If you see a new version launching, look for last year's model at a steep discount, as the internal hardware is often identical.
That 18-month window is a hard deadline, not free money. If you have any balance left after the 18 months are up, the interest rate will jump significantly, so aim to clear the total before then.