Oil Heating Upgrade for 2 buildings

Hi, All

A complete novice here so, please, be patient with my lack of knowledge...

We moved into a bungalow with oil heating. Never had one before, so no personal experience, only horror stories of how expensive to run is what I have heard in the past.
The house is well build. I think it was BER C3, although it feels warmer. The boiler is about 18 years old but working well and keeps the house warm.
Around September last year just before the heating period kicked in I topped up an empty oil tank to the brim. 850 dolares for 1.1t of fuel. I think it was a good price and so far with the mild enough winter we have used about 4/5th of the tank. Not sure how that compares to the rest of people using similar systems. The heating has one on-off switch timer that I had set up to heat about 2-5 hours a day depending how cold the week was. I am under no illusion the heating system, although it works, is efficient especially compared to the current oil burners on the market.
I rang few lads and some were gangsters, some were proper installers with paperwork and all.
I got a quote for €3700 for a dual zone heating system + €500 for proper system flush, no water tank upgrade and possibly a few hundred quid more for piping as the old system is a single zone and all pipes are buried under the floor. The price was after getting grant for whatever is available there.

I decided to push this to next year but:

We are going to convert a 77sqm building behind the house (which is approx 10m away from the house) and I’d need to get this building heated during the winter.
At first I thought about storage heaters, but then I decided to link it to the house’s heating since we are going to upgrade the oil heating anyway.

Am I correct in thinking that:

1) the upgraded oil heating system would be able to have 3 zones - main house/water/external building. I’d prefer to have it accessible and controlled via app as well as manually from the house.
2) I’d need to add 5/6 rads for the external building - 3/4 for the main room which would be 40-45sqm and 1 each in 2 smaller rooms
3) the builder can prepare all piping work to the external building and allow me to do the upgrade in the summer rather than all done at once.
4) my house is approx 100sqm and has 10 rads - all of them old enough but no leaks or rust anywhere. Should I replace them with new rads?!?
5) cost of the upgrade would come to approx 6.5k for a high efficient burner, all piping, heating controls, rads included, old system flush out...
6) the water tank is small and old and needs to be replaced.
7) am I being realistic here...