Is underfloor heating in the basement useful??

The radiators will only be about 30-40cm wide

I'm mainly concerned about the specifications

with thermostatic settings planned for basement purposes;
 a heat load calculation is not part of this offer; 
Setting up the location of the (enlarged) underfloor heating manifold;
 room thermostat control.

What do the individual points mean?

Sounds very unspecific. I would ask about the installation spacing and number of circuits.

The radiators will only be about 30-40cm wide

What does that mean? I am not the provider, so just my thoughts on this:

with thermostatic settings planned for basement purposes;

Probably set a little colder. Costs nothing extra.

 a heat load calculation is not part of this offer; 

It helps anyway if an external company makes that. Definitely have it done!

Setting the position of the (enlarged) underfloor heating manifold;

That reads to me like pointless filler text. The thing just has to be installed. Best in a vertical position. ;-)

 room thermostatic control.

Already discussed. With a proper heat load calculation including balancing, you can be exempted from that. If not, definitely demand actuator valves that are fail-open, so that the control can be deactivated.

Heating circuit distributor costs 100-300€ on the internet

The pipe 17x2 costs 200m 160€. 400m to the basement >300€.

If we calculate the "sanitary prices" as twice as expensive as on the internet, that is 1200€ material and 0.5 day of work. Maximum 2000€ minus costs for radiators.

And minus costs for the mixing device. No idea what something like that costs. Or would the HK have been operated at the same temperature?

The radiators would be operated at 60 degrees

what must be considered is that we will not have our own heating system but will be connected to a district heating network