Renovating an old building, are construction costs / effort worthwhile?

Honestly, I completely leave out the property in such cases. At these prices, the location can't be good and desirable, so I think the property itself is practically worthless.
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... or the house is worthless. Then the question arises whether a renovation is worth it. But only a professional/expert can definitively answer that. You wouldn't want it to be a bottomless pit.

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I only advised buying here because there are craftsmen in the family and among acquaintances.
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That is obviously very valuable. I notice that as well in our own projects at our house. You definitely save money. When our new front door was installed, it still needed to be connected to the socket. At first, I thought more "electrical stuff" had to be done (door control/programming, etc.). So I officially ordered an electrician (who even came on short notice). In the end, he just connected the door cable to a Schuko plug and plugged it into the socket. Cost almost 80 euros :eek: ...
However - as already mentioned - such voluntary services need to be well coordinated and planned. This applies especially to such a large project as a house renovation. No friend/acquaintance may fail here. Then there is also the question of warranty. I would hesitate to entrust some work to friends. This is also to avoid stress later on.

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That doesn't cost much in EL. With a company, you quickly lose a lot of money.

See above, front door to socket

or the house is worth nothing.

Is it still worth something!?
If you know so many people, then take someone along who is familiar with old buildings and defects. They should say something about the value of the house. To me, it also reads as if it is a tear-down house without significant intrinsic value – which does not necessarily mean it has to be torn down. But I keep saying: something like this is a lengthy and ongoing hobby. You have to want and like that. Doing it once and done is not possible with limited funds.
Am I right in seeing that you are not taking out a loan?

we are only in our early 20s, have many willing helpers

but hardly anyone said, yes buy that thing

The helpers don't seem that willing ;) I also think that a professional should first take a look at the building substance because of the moisture. If the price doesn't increase further due to the seller's appraisal, then buy.

since our budget is limited to €250,000

That is really little. Doesn't the bank want to give more?

I always find it a bit amusing when people talk about their own work and mean that they have friends and relatives who are craftsmen. One should rather calculate for such major renovations something like 75% is done by oneself, 25% is done by the helpers. These are not cosmetic repairs that the house needs. The house sounds like it was built before 1945. Given the described condition, I also assume that all water and sewage pipes need to be renewed. So, chiseling and plastering work will be necessary. Uneven floors that are clearly noticeable usually have their cause in a structural problem. It is not enough to just screw on a few OSB boards and lay laminate. Old houses also rarely have standard door sizes... It is all doable if you want. But only if you mostly do it yourself. And have a budget.

I always find it a bit amusing when people talk about own contribution and mean that they have friends and relatives who are craftsmen. With such major renovations, one should rather calculate 75% doing it yourself, 25% done by helpers. These are not cosmetic repairs that the house needs. The house sounds like it was built before 1945. Given the described condition, I also assume that all water and sewage pipes need to be renewed. So there will be chipping and plastering work. Uneven floors that are clearly noticeable usually have their cause in a structural problem. It is not enough to just screw on a few OSB boards and lay laminate. Old houses also rarely have standard door sizes... All this is doable if you want. But only if you do the majority yourself. And have a budget.

I had posted something about the topic of own contribution. I don't know anyone who does it for free. They have to be really great friends and family members for that. In our case, nobody has time! And if someone has time, they only do it for money. Maybe I have the wrong friends.

I posted something about the EL topic. I don't know anyone who does it for free. They have to be really great friends and family members. With us, no one has time! And if someone has time, they only do it for money. Maybe I have the wrong friends

It may be. For us, it is completely normal and taken for granted that we help each other within the circle of friends. Last year we redid a terrace for a buddy and built a small pool + 60 sqm terrace for another. Currently, we are helping someone who bought an older house from 1910. We've already removed several tons of construction debris there. If anyone had come to give me money for the help, they would have gotten a few gentle pats on the back of the head. Of course, it always depends on the extent, but generally as a friend/buddy you never take money as long as it's in the usual range.