It doesn't take much heat to keep a basement warm if it is mostly below ground level. The mini-split would work well in a large open space, but it could be overkill for your basement.

I think the first option is to explore using the current furnace. The fact it is located in the center of the space makes it an attractive option. It may be possible to make the basement a separate zone with its own thermostat. I suggest you call an HVAC contractor to see that is possible.

Another option is electric baseboard heating. The installation cost would be less than a mini split, but the operating costs would be higher. If your electric rates are not high then you should also explore that option.

Thank you. Making the basement a separate zone with its own thermostat would be ideal; the person living down there needs it to be about 80 F year round for health reasons.

I reside in northern IL and have a 1,000 sq. ft. finished basement, below grade, walls insulated with 2” XPS

Our primary heat is forced air natural gas. I installed two 6x12 air returns. The basement never gets below upper 60’s and that’s in the off season when the house thermostat isn’t calling for heat or A/C.

My supplemental is an electric fireplace with a 220 volt heater/ blower. Gets warm but must keep basement door closed to retain heat. Don’t recall the BTU rating

Big heat loss causes, windows, floor and exposed, uninsulated foundation.