Cost to renovate a complete house

Hello, I still need the forum's help, please. Since we still haven't been successful in finding a plot, we are now considering buying an old existing house (from around the 50s) and designing it according to our ideas. There is no concrete property yet; I am just wondering if it would make sense at all. I think everything would probably have to be redone:
Electrical system
Heating (retrofitting underfloor heating)
Controlled residential ventilation
Pipes?
Insulation?
Roof?
New windows, doors
Floors, walls, ceilings
Possibly tearing down walls and/or building new ones
...
Since I have never dealt with this topic before, I don't even know how to approach it. Do you also hire an architect or a general contractor for this? And more importantly: what costs should I calculate? I realize that the information is very sparse. We want a living area of 200m² or more. Maybe someone can still give a rough estimate? Can, for example, €250k be enough, or do I need at least twice that?
Thank you very much!

You can do it with an architect or a general contractor who specializes in this.
First, you have to find a 200m² house from the 50s; I think that's harder than finding a building plot...
With a complete renovation, you save on the shell construction but have additional costs for gutting and disposal. It's not significantly cheaper than a new build.
With high-end standards, you might be looking at 300 ... 350, plus incidental costs. With low standards, your 250 might be enough.

Surprisingly, I quickly find more offers there than for plots of land. After all, 2 of our 3 desired plots were rejected because the house was supposed to remain standing. Maybe the market is indeed bigger in that area...

Ok true, it is certainly somewhat different in the old federal states than in the new ones. Here, there is nothing like that.

As a rough estimate, you can calculate from €1,000 per sqm of living space for a complete renovation. It always depends on your wishes.

So, we have a house built in 1976 and the interior is in its original condition, otherwise a new roof covering and partially new windows were purchased. The windows would have still been okay, but they were removed anyway, partly for the sake of uniformity and partly because it then became a kfw 100 renovation and triple glazing was mandatory (theoretically). We took out a 60,000 euro renovation loan from the kfw because we were dumb and naive and on a flyer lying around at the bank, an example cost for "windows, facade, heating" was stated as a similar sum. The bank "advisor" (i.e. "seller") also didn’t think to tell us that 60k was not enough for renovation to the efficiency house standard. Furthermore, you only have 3 years for the work and you MUST reach the targeted standard afterwards. That was an enormous amount of pressure, I can tell you! We still managed it, neglecting any interior beautification and really focusing on the energetic aspects and lots of own work. The calculated example amounts did not add up at all. Just for the facade on the upper floor, the companies wanted 20k euros. We then did it completely ourselves for 5k in material costs. Otherwise, it simply would not have been enough. The sum for the windows was also easily exceeded by three times. I would definitely buy a house that is not quite that old. So not from the 50s, where everything really needs to be new, but at least from the 70s-80s, where at least the electrics are still usable and maybe some work has already been done. And I cannot recommend kfw funding at all; the conditions, the time pressure, and the paperwork are immense, and the interest rates are not much cheaper than the open market anymore. We canceled the kfw component after 5 years and refinanced it with a normal property loan. That way, you also have the possibility to stretch the work a bit and not have to redo everything in the shortest possible time.