Depends on several factors. Design of home, size, location

How do you heat now? Forced air or boiler? Ductwork system in place or none? What is fuel source? If straight electric resistance, propane, or fuel oil, then there may be some operating cost advantage to a HP mini split.

Article link below.

IMO

Here is a link that might be useful: Ductless Mini Splits

The extra cost for heating was very little on our installation so we added it even though we have in-floor radiant heating. It has come in useful a couple of times when a part on our heating system malfunctioned. It has given us breathing room to make repairs during the daytime when something malfunctions at night (which always seems to be the case).

The questions from tigerdunes are important to answer. The cost of adding heat might be small, but it does add some mechanical complexity. Reversal valves in heat pumps have been known to fail.

I have another question about your heat. Is it zoned? If not, you can heat just the downstairs. That said, heating just the downstairs tends to be difficult since hot air rises.