how long has it been standing still? I would be selfish and try to push the price down further. Maybe you are the only serious interested party
The standstill is described in the initial thread. About 2 years.
I would advise the OP not to blindly push the price down further. The OP plans: Bring in an expert.
After that, you can see where it still sticks.
Of course something is stuck after 2 years of standstill. You quantify that and then continue negotiating. If no disaster happened during construction, not much will be broken – then the price is also reasonably okay.
You have to see: The house is not tiny. The OPs plan 150K for further expansion + 145K purchase = 295,000. Corresponds to about 1800 euros/sqm. That’s acceptable.
I think they still have some potential to go down, whether 150K is really necessary depends on the OP’s requirements and the necessities to comply with the energy saving ordinance based on the completed shell (but the expert can quantify that well).
Best regards
Thorsten