Purchase of an existing single-family house prefabricated house year built 2000

Where is the house located? The square meter of land is still reasonable.

What costs should I expect approximately for the new heating system in a few years?

New oil heater or heat pump?
Is underfloor heating available?

If underfloor heating is available, the costs are manageable. —> Air-water heat pump.

No underfloor heating, therefore an air-to-water heat pump will not be possible! I will probably have to go for an oil burner, but I don’t know exactly how that will work after 2025! I would now assume a new oil burner plus solar thermal?!

Out of gut feeling, I would save 10,000 euros for a heat pump in 10 years. Inflation is high, you might be forced to use solar thermal energy in addition if you don’t have it yet and a share of renewable energy is or will become legally mandatory. But maybe there will be fewer shortages and the technology will generally become cheaper... who knows.

The 10,000 euros would probably not be a problem! The question is, can I install an air-to-water heat pump without underfloor heating at all? Alternatively, according to my research, it would only work with very large radiators?

A new oil heating system might be difficult; there are exceptions, but not many. With 70kWh/sqm, you should be able to heat the place even with a lower supply temperature, but it's not very good. The house itself seems quite expensive to me, and I claim that with that budget you could also build 115sqm new. Then you save most of the ancillary purchase costs and have modern building technology.

The 10,000 euros probably wouldn't be a problem! The question is, can I even install an air-to-water heat pump without underfloor heating? Alternatively, based on my research, it would only work with very large radiators?

Sure, you can install it, but the efficiency/costs are unknown. That's why the question about the location.
Do you have access to the actual consumption? How many liters of oil per year at which room temperature?

Is the house currently occupied, already visited? What are the room temperatures now? (Insulation/shading)