Im with Shawn. I would go heat pump or replace the wood boiler. Gas might be an option, if you have NG, but I guess not. Propane is too dirty, and not cost effective, IMO. Maybe install a few smaller units, or multi-stage setup if there are sections of the house that you want to control differently.
Is your water heater elec? You mentioned solar?
I see a flue for the oil, do you have an available chimney/flue that you can add a wood stove in the house? That might be a good option for keeping cost low, if you are willing to put in the effort (likely less than the water stove). Have a HP to knock off the chill and use a wood stove when the temps start getting lower.
My house isnt near as old as yours, but we have an older heat pump with oil backup. I also have a wood stove in the basement. I dont have the desire currently to cut and shuffle all the wood to use the stove all winter, but I run it around the clock when temps hover around or below freezing. With just the oil as backup, consumption isn't much, but when the system stops working, it will become all electric.