Hello,
How am I supposed to choose a provider if I don't even know whether what they offer is good. In principle, I would have to consult a specialist/construction advisor in advance.
How about starting with a proper initial consultation? As far as I know, you can get very good initial information at consumer centers and also energy agencies - by appointment, of course. In my opinion, that would already get you a step further. Reading here on the HBF will surely help you as well!
Furthermore, the overall impression always counts. If you get a bad gut feeling from a provider – stay away, no matter how tempting the price is. If another advisor takes the time to answer your questions thoroughly, points out items that would blow your budget, sometimes even advises against it, explains the incidental building costs (about EUR 35,000), etc. ... then you should definitely go deeper into it.
No provider – not even the cheap ones – can give you "bad" offers; they are all bound by the valid energy-saving ordinance. The difference lies in the details – e.g., only 100 mm insulation here, where maybe 240 mm would be advisable; there a careless planning => bathroom is above the living room and the wastewater pipes have to run through the living room, etc.
I already wrote this elsewhere today: if the price of the offer per sqm/living area is below €1,500.00, you can assume that the energy-saving ordinance is met at the lowest limit, the equipment must also be in the lowest price category; in the end, you will have to spend considerably more money.
Reacting with offense will not get you anywhere; on the contrary, it will more likely result in you not getting answers to your questions. So keep reading on the HBF, arrange a consultation appointment with one or both of the institutions mentioned above, and then you will see how to proceed. This will not make you an expert in the shortest time; but it will strengthen your confidence in the house-building project!
Kind regards