A good security system should last you a decade. You want a setup that uses modular sensors so you can add a window or a leak detector whenever you want. Buying a bundle upfront is usually cheaper than picking up pieces one by one. Stick to the basics first and expand as you find gaps in your coverage.
Security companies refresh their hardware every few years. Last year's model is almost always the better buy because the sensors themselves rarely change. You are mostly paying for a slightly faster processor in the base station that you will rarely notice in daily use.
The 0% interest rate is a hard deadline, not free money. If you still owe a balance after the 18 months are up, the interest rate will jump to the standard high APR on whatever amount is left.