Is purchasing a house and complete renovation affordable?

We have a heating engineer in the family. He will certainly do it cheaper, but not at a giveaway price. I myself would like to do a lot on my own with a lot of diligence and active support from family and friends. I have personally estimated about 250,000-300,000 euros for the renovation. And I have to say that I am expecting or rather hoping for state funding. That would help us somewhat out of the difficult situation with the high renovation costs. The house is supposed to be energetically renovated.

If you have no experience with complete renovations and are not skilled amateur craftsmen, then the proportion of your own work is manageable and above all not predictable in terms of time. After renovating four apartments, I can now install 30 sqm of click parquet in an afternoon, and that without any craftsmanship talent. However, for my first 12 sqm floor, I needed a whole weekend, and the floor was rather botched. And the support from friends and family should also be realistically estimated as low. If they are craftsmen, they usually have other side jobs that perhaps bring extra money, instead of working pro bono and a crate of beer. If they are also amateurs, you have to expect that the leisure activity will be brought forward spontaneously. Or children get in the way. No one works for friends for months in their free time in the dirt.

Major backlog of renovations (basement damp, new heating system needed, house and roof need insulation, roof needs repair because it is leaky, new windows need to be installed because the old ones are single-glazed and wooden, if possible underfloor heating should be installed throughout the house. A new room layout should also be done)

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But I have now heard prices for a renovation from various relatives and friends.
These ranged between 100,000 and 350,000 euros.

Well, if you really want to do everything, including insulating or dehumidifying the basement and new room layout (structural engineering), then I would go into the race with the 350,000€ renovation costs.

I would look for an architect as the next step who can help you. He can tell you what is possible and what is not, e.g. with a new room layout, underfloor heating, he refers you to a structural engineer, maybe can better estimate prices, etc. Yes, an architect is expensive, but he also delivers a lot.

No one works for months in the dirt in their free time for friends.

Unless they expect the same for their own house. Then you spend the next years on construction sites

You might forgive me, but civil servant and commercial employee... at first glance, I don't suspect any manual skills or even a "at least learned some manual work decades ago"...

Don't underestimate that. I finished the interior of my house in the last 10 weeks. Walls, floors, doors, driveway (filled and compacted rough unevenness with gravel)...
Now I haven't felt like it at all for a week, but of course there are still 60, 70 boxes standing here, lamps need to be installed, etc.

That takes a very, very long breath over many months or years. I wouldn't have that...

We have a heating engineer in the family. He will definitely do it cheaper, but not at a bargain price. I myself would want to do a lot on my own with hard work and active support from family and friends. I personally have calculated about 250,000-300,000 euros for the renovation. And I have to say that I am counting on or hoping for a government subsidy. That would get us a bit out of the difficult situation of the high renovation costs. The house is supposed to be energetically renovated

Define "a lot". We have been renovating for 6 years. But the roof and facade were still okay. I mostly enjoy doing it. Currently, however, we have been living a degrading camping life in our own house for months because we are renovating the ground floor (partly ourselves, partly craftsmen) and you have to be able to handle that. At least if you don’t have the money to do everything at once. And we didn’t have that; we are doing everything step by step. We are currently at about 1150 EUR/m2 renovation costs, but without a new heating system, without roof, without facade, and with very, very, very much own labor.