Build a gas heater today?

Even though it is already far from the original topic, if you really want to act ecologically, you should not have built.

I always find it very strange to argue on the one hand with ecology and environmental protection, but on the other hand to put up a new building that inherently leaves a huge footprint.

People are too stupid to voluntarily spend more money for more environmental protection.

That is the core of the problem.

the prices have to change.

The problem is that the Russian can currently push their gas onto us at unbeatable low prices and is by far not yet at the pain threshold on the downside. On the other hand, the costs for the power grid operators are exploding, caused by the permanent expansion of market-distorting electricity generation.
Electricity produced completely worthless on the market, as it bypasses the demand situation, has to be compensated to the producer at horrendous prices and sold abroad at negative prices to ensure grid stability. The spot electricity price will of course continue to fall. But the grid fees will in the coming years only know one direction, namely upwards, and sharply so.

if you really want to act ecologically, you shouldn’t have built.

That’s not an argument, sorry. Then I could also say, if you want to cause as little CO2 as possible, you should just die now. There is no black-and-white in this topic. That’s not how it works! Renewable energies also involve environmental damage. Solar cells have to be produced, and yes, wind turbines also bring birds down from the sky (although not as many as is often portrayed). I also want the electricity to come from the socket and not wash my machine by hand in the nearest stream. It is always an individual weighing of pros and cons. And even the complete renovation of an old existing building doesn’t necessarily have to be much better for the environment than an ecological new building. As I said, everyone has to decide for themselves. I WOULD simply no longer consider a gas heating system in new buildings these days. Mainly for environmental reasons but also because I believe that over the service life, the price difference won’t be that big and I would rather go for the more future-oriented product.

And even the complete renovation of an old existing building doesn't have to be that much better for the environment than an ecological new building.

Well, in the shell alone there are easily 2000 kWh/m² of embodied energy. In a KfW55 EPS bunker, a good chunk would be added on top of that. That's about as much energy as you can burn through, if you try, within a 35-year usage period.
So after all: renovating existing buildings is more ecological than building new ones. And the proverbial energy you put into constructing your new building exceeds the heating energy demand you will have during the usage phase.
That little bit of heating energy doesn't make the total energy demand of a new building any bigger than it already is. But with green electricity, heat pumps, photovoltaics, and ETICS you can still do some great greenwashing.

For our new build, we originally wanted an air heat pump and to generate the electricity for it and also part of the other electricity using our own photovoltaic system as well as a power storage. After receiving 2 quotes, we now have concerns. The cheapest quote for a heat pump with 5.08 KW, photovoltaic system with 5.1 kWp and storage with 5.12 kWh is €33,000. Should we rather take a conventional gas heating system? That would definitely be cheaper. Or is that absolutely no longer up to date?

Do you actually have a cost comparison for us? There is already a wild discussion about consumption, costs, ecology, etc. here.

1. What does a heat pump cost without photovoltaic?
2. What does a gas boiler cost without heat pump including gas connection and exhaust pipe?
3. What does a reasonable photovoltaic system cost (max. 9.9kW without storage)?
4. Is the offer including underfloor heating with all accessories?

After that, we can give better answers.

An argument consists of a standpoint and a justification.

Standpoint: those who truly act ecologically do not build new
Justification: a new building generates a huge ecological footprint

As you can see, this is definitely an argument, which you have not refuted.

I would like to ask you to refrain from rhetorical tricks.