Which heat pump? Ventilation system / Air-to-water heat pump

In general, groundwater pumps are a matter of what kind of groundwater you have, that is how suitable it is (maintenance costs) or if it is even feasible. Then you have to obtain a permit. Efficiency and acquisition costs are okay if the parameters above are met.

Is it necessary or advisable to have a test drilling done in order to immediately check the actual water quality and flow rate? I once read that, but 2-3k € for it is not insignificant either. Of course, you can then use the hole for one of the two wells, but it could also turn out that it is not sensible to use a groundwater heat pump....

It can also happen with a geothermal drilling that the drilling does not succeed (e.g. cavities that make grouting impossible or similar).

But there it is even more expensive or the financial loss due to the deeper drilling is even higher. Is something like that very rare or does it happen quite often?