How to plan heating with a heat pump in new construction?

It is slowly becoming more concrete with us now, and the architect is obtaining initial offers for the trades. Among them, of course, is an offer for the heating system, and I am now wondering what to do with it. I have absolutely no way of assessing whether the offered system is correctly sized and the price justified, or if there might be a much better alternative.

A ground source heat pump with an outdoor unit is definitely to be installed. If the money is enough, we also want to build a photovoltaic system with battery storage right away, but if necessary, this should definitely be done later. You will probably have to consider quite a few things directly with the heating system in that case. The house will have a living area of about 155 sqm on two full floors, all underfloor heating, and it will be occupied by four people. What else should be considered for the heating system?

The first offer is as follows:
Buderus air/water heat pump package WLW196i.2-6 AR with heat pump manager for outdoor installation, heating output 7.3 KW, immersion heater, 190 l hot water storage tank, connection accessories, circulation pump, pump group for heating and hot water operation, heating circuit connection set, boiler safety set, cap valve, 120 l buffer storage tank, membrane expansion vessel, circulation pump, shut-off valve and piping, including 8m connection line from outdoor and indoor unit through a pre-installed empty conduit, heat pump prepared for cooling with humidity sensor and dew point sensor, delivery and installation: €19,468.30
Of course, in addition, all the preparation for the underfloor heating, insulation, control, ...

Now I have a lot of questions. Is this a good system? Is the heating output correctly sized? Is the hot water storage correctly sized? Is the buffer storage correctly sized? Is this a reasonable price? When I search for the system on Google, I find offers of under €12,000, for example. But of course, I don't know if everything listed above is included there and whether you can compare at all.

I would like to better understand what I need to pay attention to and how to proceed best. This trade is one of the larger and more important items, where one might quickly spend too much money or simply bet on the wrong horse.

What else should be considered regarding the heating?

The building envelope, for example? It is much more important than the number of people. You are probably somewhat oversized with 7.3kW. The heating load calculation will give you more precise information. A buffer tank is counterproductive with a heat pump. Whether 190 liters of hot water is sufficient depends on your showering and especially bathing habits.

What was the result of the (Raumweisen-) heating load calculation? If nothing at all, it certainly makes sense to calculate it before ordering a heater.

If you start without a planner, you can already give the heating installer a few specifications.

E.g. flow temperature maximum 25-28 degrees, with that you already sort out 80% of the bunglers who say "we've always done it that way."

Laying distance nowhere greater than 10cm, walls are mandatory in the bathroom, pipe length maximum 80m.

And with 7.5 kW you can heat a 300m² house on the Swabian Alb to 25 degrees. So way too big. Better to do a room-by-room heating load calculation based on realistic values. That is 24 degrees in the bathroom, 22 in living rooms, and 20 in bedrooms. You should reach these values without additional heating in the bathroom.

For a 150m2 house you end up at about 4kW. So the modulating range should be 1.5 - 4 kW.

Regarding the buffer, I would make it dependent on what kind of bathtub you have and how long you shower. We have 300 liters, are 4 people plus mother-in-law in the granny flat, water has never run out unless I played around with the buffer :)

And don't let the heating installer tell you it won't work, he just wants it comfortable and quick, but you pay the bill in the future with your electricity and that won’t get cheaper.

The claim that electricity will not become cheaper is a bold thesis that is heard more and more often, but is not supported by anything at all.

When heating becomes 10, 15,000, 20,000 EUR more expensive due to partly absurd requirements like a maximum flow temperature of 25°C. When do you save that back again? You have to front the money for the coal first...

Thank you very much for the valuable tips. I will now inquire about the heating load calculation. The 7.5 kW also seemed very high to me, as I had always read about approximately 5 kW for this size range here in the forum. What is the deal with this buffer tank and why is it counterproductive with a heat pump?