I would strongly advise against purchasing.
Has the house been thoroughly inspected? A nice facade can hide a lot. Why was the bathroom completely renovated after 18 years? Was something leaking? In a prefabricated house, that is deadly. Not to mention the possibility of mold or worse in the ceilings and walls. I would investigate that first.
I wouldn’t count on 450 € for operating costs anymore. It will definitely be much higher. 80 euros for electricity is unrealistic. Costs for heating oil, currently about 1.50 euros. How big is the oil tank?
You will have to expect at least 150-200 euros for electricity. I also think about 200 euros minimum per month for heating oil. Plus firewood. Yes, that costs money as well. Property tax, insurance, water, sewage, garbage, possibly street fees, that adds up quickly.
We are easily talking about 700-800 euros in monthly maintenance costs. Maybe not this year, but probably next year. Grocery costs will also be difficult with just 100 euros per week if you don’t want to live on oatmeal alone.
You can keep the heating system. Oil is currently the cheapest heating method.
But despite all circumstances, I find your approach difficult. The house feels too small and too expensive. Regardless of the plot. A prefabricated house is something special and experiences higher depreciation than a solid construction. That doesn't happen without reason.
With such a high amount of equity, I would simply wait. Cash is king, and we might be on the verge of a deflationary shock similar to the Great Depression of 1929. Probably even worse.
You can already notice the lack of liquidity everywhere. Wait a few more months and you will be able to pick houses with a 30 percent discount. You are still very young, without experience. There is no reason to panic at the last moment.