Anybody here with experience buying and completing an unfinished home?
No, but we built one, so I know that from the 'raw' state there's still an awful lot to do.
400 m is huge. I'd say you'd need at least 350k to finish it to a reasonable standard. (I don't envy you your fuel bills in a five month Polish heating season)
Do I need any building permits to complete? Any inspections along the road now?
The builder will already have obtained permission, but you still need a building manager to inspect and sign off each step, e.g. utilities installations, stairs, fireplaces. He/she will usually charge you each time a step is completed.
home has been empty for 10+ years - do I need any formal inspection to re-start the project?
I don't know the answer to that, but if I were you, I'd get a building manager to do a thorough inspection before you buy.
Our little house (just over 100m) cost about 250K, most of which was spend after the walls and roof were up.