That naturally depends on the region and that varies. Unfortunately, I cannot provide any prices either. I think the two storage units are sufficient and I would not expand that further.
I also think the capacity of the boilers is sufficient. About 100 meters of drilling depth is already necessary. However, that depends on the soil and what performance is to be achieved.
Unfortunately, I have no experience with pricing and therefore cannot write anything about it.
Drilling depth approx. = cost for drilling (flushing head)?
It is important that the heat pump fits the house. Often, some mistakes are made in this regard. As I roughly estimate it, it is well dimensioned with Martin11.
I had obtained some offers for the drilling costs before. The cheapest was 250 CHF for 10 meters of drilling.
I also think your capacities are sufficient. I am planning something similar. I have obtained quotes from companies that carry out these deep drillings. The costs are similar and range between 230 and 260 CHF.
This is a kind of base price. It can become more expensive if there are difficulties. I just hope that the subsoil is not too porous.