Hi Lemon,
Im in the same boat, I bought a house built in 1979 and I have an oil Stanley cooker in the kitchen also, I have done lots of research so I will help as much as I can.
I rang Staley and they told me my Staley was over 20 years old and they don't built them anymore this type of Stanley I have eats the oil and they are not sufficient, a new Stanley will cost any thing up to 7000-10000euros. this is too costly so I suggest doing what I am doing.
Get a plumber in to quote for the following.
1) A New boiler for outside the house e.g the back garden they cost about 600.00euros for the boiler
2) Rads cost anything from 70-100euros each and then you have to fit them also which will cost something like 150.00euros max per rad supplied and fitted, it is up to your father if he needs new or more rads, however if the rads are old they take some time to heat up new rads have one third of the water inside them so they will heat up quicker causing less usage on the boiler there fore saying you money on oil, new boilers will save up to 20% versus an oil boiler, new rads are more sufficient but are not necessarily needed to be changed
I got a price of 3,500.00euros + vat this includes a bath to be fitted into my bathroom so your costs should be less, parts cost about 2,000-2,500euros alone, heat merchants is where you want to go to get costs for everything but unless you know about fitting a boiler and rads I would leave it up to a plumber. Prices may be different from each plumber you ask best to ask for a couple of quotes I would think.
Best of luck