As an aside - the place already has a lease to some fantastic tenants who are willing to extend the move in date a couple of weeks.

The sellers want to lease back, while they find a home to move into, and we want to rent it to them, since we realistically don't want to move for another few months. 

A 3rd option may be a lease with the option to buy. 

Add a line to the lease that states your process on this.

You want to confirm rent, security deposit amount and all terms of the lease. 

Review their lease agreement and addenda.

The heat exchangers fail most frequently on the propane boilers due to the high acidity, low temperatures the propane condensing units run at, my friend who owns a 3rd Generation heating/plumbing company doubts those units will ever come close to outlasting a traditional 3 pass boiler.Agree the pros of having a propane boiler/furnace-fuel is cleaner, services tend to be cheaper, tanks can be bought or fuel company can supply tank, work great if you also have a tankless hot water heater with same fuel source.Efficiency of Maine does offer rebates as well for the high efficiency units, they don't mention this but, if you insulate all baseboard piping that is exposed in the basement, take photos and have a high efficiency oil boiler they will qualify as the same rebate as the propane unit.

Or I may just keep leasing it.

The switch is a PITA and a notorious problem with other well owners down here because the switch is outside and exposed to the elements and fireants and frequently fail.

As a beginner I would look at wholesaling or lease options.