I am not sure what the units themselves cost, but to satisfy the mortgage company, you might want to consider electric baseboard heaters. They are simple to install, so you could save time & money. After closing, and when you have the funds, you may want to consider doing additional ductwork and/or upgrading your furnace. The electric baseboard units are a little expensive to operate, but if you aren't really planning on using the rooms, you can turn off the heaters and close the doors. Some of the units have thermostats (something the mortgage co. may require?). I would get heat requirements in writing from the mortgage co. before you do anything. I would hate to see you do something, even if temporary, only to find out that it doesn't meet the mortgage co's requirements.