ᐅ Our First House – We Need Your Advice!

Good evening, dear community!

I have been a silent reader for weeks but now need expert opinions and fresh ideas as we can’t reach a consensus on our issues.
This will probably be a very long text, so please forgive me if I’m asking questions that have already been answered elsewhere or are misplaced.

So.
We (me, my wife, and my parents) have the opportunity to buy a house from a close relative.
It is a detached single-family house with three floors? (basement, ground floor, attic). The footprint is 9 x 13 meters (29.5 x 42.6 feet) on both basement and ground floor. That means approximately 230 m² (2475 sq ft) plus attic space.
I estimate the lot to be about 500 m²+ (5400 sq ft+). I haven’t received exact numbers yet.
Year built: 1987/88.

What needs to be done?
- The house has wooden windows installed — so new windows are needed, but which type? PVC (vinyl) or wood/aluminum composite?
- The interior condition reflects about 30 years of use. A few tiles are broken here and there, door frames damaged. Bathroom dated in ’90s style.
Basically, we want to renovate EVERYTHING inside: walls, electrical wiring, plumbing, flooring, bathrooms.
- When I inspected the roof from the inside, I could see through to the outside. So it is not fully insulated.
- As far as I understand, the house has no insulation. Maybe the attached documents clarify?
- The house has no energy performance certificate.
- The heating system also needs to be replaced.

We want to buy the house because it represents the "final" step in terms of space enlargement.
My parents want to occupy the entire basement, and my wife and I would take the rest.
We want to separate the basement as a granny flat (secondary dwelling), also considering the possibility of renting it out in the future.

Now to the basic problems and questions.
My father insists on having one boiler/heating unit per dwelling. To better calculate the consumption.
My question to you: Is this common practice?
Is it true that these boilers require an air supply and exhaust flue similar to a chimney? During the renovation, some walls will be removed. In the basement, the boiler room (please see the attachments) would connect to room 01Child to create a larger living area. Room 04Play becomes the kitchen without a wall to room 01Child; 08Cellar turns into a bedroom. The small bathroom will be removed and a new bathroom created in room 03Cellar. I’m not 100% sure what will happen to 07Cellar.
On the ground floor, walls will also be removed. Rooms 2Child and 3Living will be combined; rooms 7Bathroom and 5Child will form the new bathroom. Room 4WC will be removed completely.

How exactly would one integrate a boiler in this configuration if my assumption above is correct? Am I right in thinking that these boilers are those tank-type units hung in bathrooms/kitchens, like in caravans or older apartments? That would be a complete deal-breaker for me.
Is it possible to run an exhaust duct several meters through the apartment up to the chimney? Or do you have other ideas? Please feel free to get creative!

Next problem. In the entrance area on the ground floor, there are two spiral staircases. One goes clockwise upwards on the right, the other, located deeper in the room, goes clockwise downwards. Is this description correct?
To define a secondary dwelling, you need separate residential units. However, if the status remains as is, I would have to install a lockable door for the ground floor and attic to prevent my father from simply walking upstairs and entering my bedroom. I consider that a bit absurd — I hope you agree. The door for the basement apartment will likely be near the last few steps of the staircase.
I hope this problem is explained clearly. I’m also asking for suggestions here!

The attic is largely unfinished except for one room and is currently used as storage. Here I really have freedom to design the rooms as I like. A bedroom with a walk-in closet, a small toilet, a small hallway, and 2–3 children’s/guest rooms should definitely be feasible. I already have some ideas, but I would greatly appreciate any floor plan suggestions!

From the outside, the façade shows some spots where it looks like something dripped down, best described as a stain on a T-shirt that has dried. Could this be moisture or leaks in the roof?

As mentioned, the house apparently has no insulation. Is it worth adding insulation now? Is it still worthwhile?

Regarding the heating system, I don’t know much. Except for room 3Living on the ground floor, every other room has radiators. Room 3Living apparently has underfloor heating. What would you recommend?
Complete underfloor heating for the whole house? Using liquid medium? Electric? Underfloor heating combined with radiators — is that worth it?

There are also some outdoor tasks. The terraces need to be leveled, there is a fish pond, and the path needs attention, weeds, etc.

Now for the numbers.
At the very first discussion, the seller mentioned a price between €180,000 and €200,000.
She now wants to have the house formally appraised. The property is located in Märkischer Kreis.
From my preliminary calculations, I expect renovation costs between €100,000 and €120,000. Is this realistic?

Recently, I went with my father to the local savings bank. On our side, we would include two condominiums (one fully paid off, the other with 8 years left on the mortgage) with rental income. We will cover the purchase incidental costs ourselves.

Additional questions:
Do I need to involve the building authority to convert a single-family house into a single-family house with a secondary dwelling (granny flat)?
Should only my wife and I be listed on the title, or all four of us?
Does an architect need to be engaged due to the extensive structural changes?
Is it worth applying for government support programs (like KfW) given the scale?

I hope I was able to explain my concerns understandably and thank everyone who has read this far.
I am already looking forward to reading your ideas and suggestions.

Good night