Best heating solution for separate utilities

[Conte Cuttino]

Im gutting a Mid to low income area single fam home with an accessory Apt. It will need a new HVAC system. It previously had a forced hot air w/oil furnace. There’s no gas in the street. I want to keep the heat separated for each unit to pay its own utilities. What’s the most cost effect solution. FYI: I’d love to not have to deal with oil anymore.