Experience with renovating old building condominiums

Hello everyone,

first a brief introduction: My name is Artur and I have been a silent reader for a long time.

My wife and I have found our desired property and are currently in the final stages of the purchase process.
It is an old apartment (5 rooms + kitchen + bathroom + balcony) with about 110 sqm of living space in a 3-unit multi-family house from around 1905 in a Lower Saxony major city.

The apartment was last completely renovated in the late 1990s.
Some things are still acceptable to us (bathroom, for example), but of course we want to implement and refresh our own ideas.
I would appreciate your support with this!

First of all: Should I post my questions separately by topic in different sections? Or collect everything here in one thread?

Walls
The apartment is completely wallpapered with woodchip wallpaper in the living rooms.
We will remove the wallpaper and want to leave the walls simply plastered and paint them with silicate paint.
As far as I understand, silicate paint only adheres really well to lime plaster.
Of course, we do not want to knock off all the existing plaster down to the masonry but want to get the walls as diffusion-open and reasonably smooth as possible with as little work as possible (they do not have to be super smoothly filled).
How do I determine what kind of plaster it is and how should I best proceed?
What prices should we expect if we want a professional to smooth the walls with filler?

Fireplace
The current tenants operate a small stove in the living room.
My wife and I want to continue using the stove in addition to the normal heating system.
Can we simply continue to use the tenants’ old stove (grandfathering protection)?
If the tenants take the stove with them, what do we have to pay special attention to when buying a new one?

Kitchen
We have to redesign the kitchen.
We hope that the old terrazzo floor is under a PVC floor covering. If not, new tiling will be necessary here.
My wife and I want tiles with geometric patterns in the Art Nouveau style – fitting for the old building.
Can we simply buy such tiles and then tell a tiler "Please install these tiles"
Or do craftsmen no longer do this today because of liability issues and so on?

Thanks in any case in advance!
I have surely forgotten important details and will try to provide them as well as possible.

If my post is in the wrong section here, I apologize!
Where should it correctly be posted?

First: Should I post my questions separately by topics in other sections? Or gather everything here in one thread?

The apartment was last completely renovated in the late 1990s.

What prices should we expect if we want to have the walls smoothly plastered by a professional?

Can we just buy such tiles and then tell a tiler "Please install these tiles"? Or do craftsmen not do that anymore nowadays due to liability and so on?