Buying/remodeling an affordably built house, what should you pay attention to? Costs?

Hello,
we have found a house in our absolute dream location.
It has about 125 sqm of living space, an 850 sqm plot, built in 1998, cost 110,000 €.
Unfortunately, the house was built quite "cheaply." Currently, there are smoking, trashy (sorry, but true) tenants living there.
We will probably have to renovate/refurbish and also want to implement our own wishes.
I am quite familiar with development plans, etc., and we will go through the house again with an expert beforehand to avoid really nasty surprises.

The following measures are planned (starting with the roof):

-Pigeon spikes and lightning conductor on the roof
- Re-insulate the house (roof with little to no insulation)
- Renew bathrooms upper floor + guest bathroom ground floor (solid, without frills: upstairs tub, shower, toilet, 2 sinks, downstairs toilet + washbasin)
- Brick up one door on the upper floor to open another one (keyword: walk-in closet)
- If financially possible, add a large dormer (If I saw correctly, a prefabricated dormer of this size costs 6,000 €. What needs to be considered during installation? Is it better to build it yourself, where are the weak points?)
- Replace floor coverings in the attic, preferably wood
- The staircase should be renewed (simple wooden staircase, straight with a short 90° bend at the end)
- Replace/renew ground floor tiles
- Underfloor heating if possible
- Probably remove a non-load-bearing wall (of course we will clarify carefully)
- A ground source heat pump with solar and photovoltaic (50% paid by my parents as a wedding gift)
- A garage
- Garden renovation

In our close circle of friends and family we have:
a bricklayer
a carpenter
a joiner
a tiler
My husband also has experience in house construction.

Of course, we want to work ourselves as much as reasonably possible.
We will contract special things like the heating system and whatever else is connected to a company.

The house was unfortunately built quite "cheaply." Currently, there are smoking, ass-like (sorry, but true) tenants living there.

I don't really know what you mean by "ass-like"... possibly a misuse of "asozial" (antisocial)? If that is the case, and you mean irresponsible people who do not really take care of their living space and treat it thoughtlessly, then it might be that these tenants are not exactly keen on leaving the house.

That could become a problem for you.

I don't really know what you mean by "assig"... maybe "asozial" used incorrectly? If that's the case, and you mean irresponsible people who don't exactly take care of their living space and handle it thoughtlessly, then it could be that these tenants are not exactly eager to leave the house.

That could become a problem for you.

Yes, in our region people like that are called "assig". I agree with you. However, they know that the house is to be sold and that they might have to leave. I have already prepared myself for a "fight" and am aware of the problem financially as well, and prepared for it. :-)