I own a duplex with 2 units, 2 old boilers that won't last much longer, i want to sell?

the house, until then i wonder if i should do something about the heating situation before winter comes, boilers are expensive and i would need two, should i put in a electric water heater and maybe oil fueled monitor heaters or propane heaters, i am limited with walls and space for venting? does anyone know how much a hot water boiler should cost?