What to replace an oil heater from 1989 with in an old witch's house?

You could heat bivalently. Keep the old heating system and use it from, for example, -2 outside temperature upwards. Everything above that with a heat pump. For example, the Panasonic that a user here has.

Hmm, to support the heating, I would prefer a wood stove in the living room. A heating engineer estimated the cost of a new oil heating system at 20,000 euros. The heat pump would probably be quite a bit cheaper. I am currently calculating around 8,000 euros for material costs, 8,000 for installation/removal/disposal. If it is then replaced in 15-20 years, the effort should be manageable. From the difference, I could already heat quite a lot again. Ideally, my system is still eligible for subsidies, then the difference will be significantly bigger again.

I would then invest the saved amount directly in photovoltaics / better windows / better radiators possibly some insulation and thus ideally also further increase the living value of the house.

For example, the Panasonic that a user has here.

Yep. Price/performance unbeatable. Experience report in 4-6 weeks.

The heat pump would probably be quite a bit cheaper.

Take the monobloc. You can simply connect it yourself to the old heating pipes.

Hmm, that would mean I have to lead the water pipes through the house wall to the outside. If the heating then fails and it is winter, I believe that would quickly be quite suboptimal? Where is the big advantage of the Monoblock? For the split units, you no longer need an annual leak test.

Tell me, what is included in 20k for a new oil heating system? That seems SIGNIFICANTLY too expensive to me?