Renovate gas heating - costs?

Ok, I understand - then you can hardly just have offers sent to you but rather have to bring the people on site.

What am I supposed to say about that now: Well, I have received written confirmation of all our offers by mail. That is not insignificant later on if it turns out to be a bit more expensive. It should be clear that someone should take a look on site beforehand.

Sure, and I agreed with you, so you didn't have to say anything else ;)

...Euro, of course it’s about the investment, as with the other figures I mentioned.

Clearly one-sided view. For this, you could use the cheapest gas condensing boiler available on the market. If the actual demand (power, energy) for heating and hot water is not known, the chance of going "down the drain" is relatively high. In old systems, the actual annual efficiency is usually 40..60%, meaning 40..60% of the energy used brings no benefit at all, only costs ;-) Suppliers charge based on condensing heat, not calorific value. That alone theoretically accounts for 11% of the energy demand for heating and hot water! If the condensing heat is not optimally utilized, theoretically 11% is lost with "zero" benefit. Theoretical efficiencies (colorful advertising brochures) of ~85% are practically never achieved even in new systems! With optimal design, sufficient planning/dimensioning about 80%, but then everything must be right! Often even those hover around ~55..65%, whereby the sustainability of the new investment should be questioned. Now calculate the consumption costs over 20..25 years, with an energy price increase of ~5%, then you’ll know what a senseless, ad libitum chosen boiler for 1,000 € is actually worth ;-) best regards

Euro, on the one hand you say that the old systems only achieve a low annual utilization rate, on the other hand even new ones do not reach the specified efficiencies. That would argue in favor of dispensing with an elaborate (condensing) technology; with the boiler, it's not done. In the specific case, it is also conceivable that the apartment is rented out, so the possibly 20 years of more economical use do not benefit the investor.

Euro, on the one hand you say the old systems only achieve a low annual utilization rate, on the other hand even new ones do not reach the specified efficiencies. ...

Utilization rates are energy-related, efficiencies on the other hand are power-related momentary snapshots. There is a world of difference!

....That would argue for foregoing a costly (condensing) technology; with the boiler it’s not done yet either....

New investment in condensing technology is worthwhile, but only if the condensing benefits can actually be realized. With new systems this is not a problem, here the technical building equipment planner designs heating surfaces and hydraulics accordingly, provided they are professionally competent.
In existing systems it looks completely different! A simple, thoughtless replacement could become a money pit.
Moreover, one should clearly differentiate between owner-occupation and rental. In owner-occupation the permanent ratio of investment expenditure to consumption costs is important.
In rental (cold rent), the consumption costs incurred by the tenant are rather uninteresting to the landlord ;-) From an economic point of view, investing here would be almost foolish ;-)

Regards