Water-carrying fireplace or wood gasifier - experiences?

Believe me, the water management will never be profitable.

What does a water-bearing wood stove (that is, a wood stove with a water jacket + hydraulics + buffer storage + adjustment of the heating control) cost in additional price compared to a simple wood stove / Swedish stove nowadays?

Back then (2009) with us, it was about €5000-8000 extra. And then no one wanted to take on integrating the stove with the main heating system (air heat pump).

Heating engineer: none of my business - I take no warranty for trades I don’t do, besides I can’t make use of the big buffer storage...

Stove builder: I’m not familiar with every heating type and heating control, that should be done by the heating engineer themselves....

It quickly became clear to us that the two trades don’t like each other – from the start.
And for the extra cost of the water jacket, you can buy a lot of wood or run the main heating...
In the end, it became a normal Swedish stove without any frills....

Since you want a gas condensing boiler anyway, you have to put solar thermal on the roof. Solar thermal doesn’t require a south orientation or direct sun.
Skip the water jacket and instead put a few more solar thermal collectors / tubes on the roof and support the heating that way....
And if you ever have to do wood (clearing storm damage...) and don’t need the wood, a two-liner with the photo on eBay Classifieds is enough and the wood is sold. In return, you can buy gas.

P.S. I am a committed photovoltaics fan.
What pitch should your roof have? Under 25 degrees, you can very well equip east/west/southwest roofs with photovoltaics. Even then, a photovoltaic system usually pays off after about 10 years.

Does anyone happen to have a wood gasifier in the utility room and can tell me something about the heat radiation? Food is also supposed to be stored in the room (HV is supposed to be behind a small partition wall). Furthermore, I would be interested to know approximately how high the annual consumption is. As mentioned, the plan is a KFW 55 house. Would about 7 RM be enough?

7 rm will get you through the winter. In terms of energy content, it fits with around 14 mWh. But you will also need hot water, and you will then generate that in the summer using another energy source. I only know large wood gasifiers for workshops. They are hardly insulated and therefore completely unsuitable for a utility room. I don’t think you want food and the gasifier in the same room. Your utility room is surely also KfW55...

So it gets quite comfortably warm in the room where the carburetor is located. However, you should always keep a window open. Additionally, dust naturally accumulates and it is not completely odorless. Sufficient wood storage space should also be available there. I have about 8-9 RM of storage space in my heating room. Basically, I would not recommend a utility room for this purpose.

That sounds like you really have to want it badly...