It's fairly easy to blow insulation into the attic as well as existing exterior walls. You can also add foam board insulation to the exterior of the house. This makes the walls thicker of course but also is a good time to replace old windows.

Should make a noticeable difference in your comfort level and energy costs

If you can get assistance/funding, you may look into a mini-split heat pump. Can buy a 12k unit off ebay for $425. A 12k btu unit is rated to heat and cool about 650 SF. They draw a little over 1000 watts at peak load. More efficient than an electric forced air system. You can install yourself in a couple hours. It is basically a ductless heating and air conditioning system. Somewhat similar to that of what you see in a hotel room.

ol'dad

How do you add the refrigerant once it is installed?

Most of them come pre-charged. You will need to vacuum the line once they are connected. You can DIY with a $65 set of harbor freight gauges or probably pay a local HVAC tech $100.

ol' dad