The ground source system can be retrofitted. Instead of vertical wells, installers are now plowing horizontal pipe with a Ditch Witch. A relative put one of these in via a pond and it does work well. We live next to a lake but didn't want to chance someone’s propeller slicing into the pick up source.
It’s just me and a dog in a 1600 sq ft ranch right now, electric heat pump keeps the house relatively comfortable and it’s not that expensive. Really hot or really cold (by our standards that would be single digit lows) the bill can creep up over $150 a month but generally it’s less than $100 a month. I don’t think for a minute that the current rate is going to stay put though, so I’m always looking around. Those far infrared film systems do sound intriguing, since I’ve always loved radiant heat. Just put it in beneath the flooring of whatever type, tile, wood, carpet, whatever. Thermostatically controlled just like anything else.