Smart thermostats are about comfort and stopping the waste of heating empty rooms. A good setup with sensors for two zones should last you at least a decade. You want a system that talks to your phone and learns when you are actually home.
Buy in the fall when utility companies offer rebates and brands refresh their lines. Last year's model is usually the better buy because the hardware rarely changes even when the software gets a minor tweak. A solid two-zone package usually runs about $350.
The 0% intro APR for 15 months on purchases and balance transfers is a deadline, not free money. If you still have a balance when those 15 months are up, the standard interest rate will kick in on whatever is left.