Is $15,000 a good estimate for ductless heating / AC in 1940s 2-story home?

The ductless AC systems essentially put an air handler in each room of the house or at least in each main area. For older houses where you can not actually put in ductwork the ductless systems may be the way to go. As to heat pumps being efficient in colder weather. The newer style heat pumps are only efficient down to around 30 degrees so yes the most bang for your buck would be a combo of heatpump with oil/gas radiant heat. This is uming you will be keeping the house more than 5 yrs. Really 15k for a 1940s existing 2 story house is not that bad. It is of course advised to get at least 3 estimates which are free from any reputable dealer out there.