A solid mirrorless kit is an investment in your creative workflow that should last you several years. By picking a versatile model now, you avoid the trap of buying cheap gear that you will just need to replace in six months.
Camera brands usually drop new models in the spring, which means retailers dump last year's tech to make room. If you buy a slightly older model during these sales, you get pro-grade specs for a fraction of the price of the newest release.
The 18-month window is a deadline, not free money. If you have any balance remaining after that period, the bank will start charging interest on the debt, which will add significantly to your total cost.