Hhhhhhhmmmm, there are a lot of unknowns here. Not a good way to judge anything. To put it in software engineering terms, GIGO.
What is the rating of the oil-filled heater you are thinking of using? The best seller on Amazon is only 650W (my toaster is 1200W!).
I bought one a few years ago but no longer use it (moved house). It's rated 3kW. Low is 1.5kW.
To run this on low for four hours is 6kW, i.e. 6 units. So currently it would cost 6 x €0.1376 (ESB price) to run it for four hours (per day); €0.8256.
Six months, say 182 days, would cost €150.2596 per annum.
Cost of new porch is €3,000.
Cost of new doors & windows is estimated to be €2,000.
Assuming that the price of ESB electricity will not change (neither up nor down), a 1.5kW heater will cost you €4,598.55 after 33 years.
In fact, after 20 years it will cost you
€3,005.19. More than your new porch!
Personally I would make no decision on my above calculation because I didn't take electricity price inflation into account. I'll hazard a guess that in 20 years one unit of lekkie will cost more than €0.1386.
Did your CBA index link the price of electricity?
Buddyboy, what about you?