Is purchasing a house and complete renovation affordable?

You can look at it one way or another. I have more than double the plot size and still have a building plot for the children in the back part

OK, such considerations really shouldn’t play a role if there aren’t even any children yet (and you never know where the children will end up later). It could just as well be that the children will move out at 18 and want to go far away because they want nothing to do with the house anymore when every cent had to be turned three times to pay the loan. I read a lot of panic and pressure and FOMA there. But for the biggest financial decision of your life, you should allow a rational view and not be driven by the idea that you have to have a HOUSE in exactly THIS area NOW to be happy. And your setup can be quite a strain on a relationship. You are still young, not married, and want to bind yourself to such a burden? And you are planning children in 3-4 years who will significantly reduce the family income for years. Even from the point of view of the resale value in maybe 10, 15 years, that thing would be too risky for me. My wife and I bought our first apartment in our early 30s. We were able to pay off the apartment with an inheritance and decided on a house as a renovation project two years ago.

But the friend got the better deal. 136 sqm new build with well-laid-out rooms with a nice floor plan is often better than 160 sqm old building with crooked and small rooms. And that at 1% interest.

There are a lot of assumptions in that. We neither know at what price the friend financed nor do we know the floor plans. By the way, the OP does not have 160 sqm, but only 130.

but this is mostly a forum for new-builders and accordingly the response is like that.

Although, rationally one has to say... 330k for the renovation of 130 sqm is quite something. That is 2500€/sqm and the "shell" is already there. This only makes sense here because the OP more or less pays the land value and practically gets the house for free.
If one had to buy on the free market and there would still be 100-200k due for the house, the new build would certainly be cheaper. The prices for old buildings are still way too high if you more or less honestly calculate the costs of a renovation.

There are a lot of assumptions in there. We neither know at what price the friend financed, nor do we know the floor plans. By the way, the OP does not have 160sqm, but only 130.

However, one must rationally say... 330k for the renovation of 130sqm is quite a lot. That is 2500€/sqm and the "shell" is already standing. This only makes sense here because the OP is more or less paying the land value and practically getting the house for free. If he had to buy on the open market and would still owe 100-200k for the house, the new build would certainly be cheaper. Prices for old buildings are still far too high if you honestly factor in the costs of a renovation.

Yes, that’s true, but this thread is about a specific project and in this case it initially seems quite promising.

The 50k of personal labor initially sounds tempting. I've put about 25k of personal labor into the house myself – so if I had hired a trade to do it, it would have been 25k. Yes, of course I had to buy equipment. Tools. Phew, that adds up quite a bit. And the tools too – well, they stay in the house. I haven't used them since the house was built. What the professionals do in one day, I spent a whole week on; during the day in the office, I fell asleep in meetings from exhaustion, and my colleague looked at my somewhat scratched hands and asked if I had been pulling cables again yesterday. So out of the 50k of personal labor, only a part remains, and it was hard earned.

When it came to boxing in the wastewater pipes in the basement, I just watched how the drywallers worked and considered for 0.1 seconds doing it myself – until he told me the price and I thought: For that money, I couldn't even buy the materials.

and having to maintain the whole garden in old age is no fun either… better to have 300 low-maintenance square meters

Low maintenance is less a matter of size and more of the layout. If everything is supposed to look as if the gardener was there yesterday... And the other question is whether with 290sqm minus house, parking spaces, terrace, paths, garden shed, the garden can actually still provide what one expects from a garden. There isn't much space left, nor privacy. It's effectively a balcony with a few bushes.