We have been running across this issue and here is how we have solved it.
First- it depends on local code, inspectors etc.
Texas- where we are in Texas- DFW area and in The Valley, we only need to supply a dedicated outlet under one window in the main living area, and the master bedroom so the tenant can put in their own ac/heat unit. So we have been converting homes to electric only.
Everywhere else- We have been using 110v baseboard heaters in each room, with unit mounted controls. You have to hunt for them, but they cost (in general) $120 per unit. We will put one in the main living area, maybe one in the kitchen, one in the bath and each room gets one. Read the box, a small heater will provide LOTS of heat. So know your SQ Footage. This again the home off gas. The heaters come in baseboard and also some wall mount styles, so do some research and shop around.
Also- in the winter we use the portable electric baseboard types for rehab projects. You can move them from room to room, and they will bring the temp from nothing to 70, in like nebraska, all the way up in an hour. That is a 6’ model that is 110v. working under the home- if it is dry- and you have skirting on- this is a great option if your working on water or sewer issues- keep the heater high, and warm the space for an hour prior to working under the home. Your frozen, cold contractor will get the job done much faster…