Why would this structure lead to any legal problems / so much additional accountancy cost? It is quite a common structure in the Netherlands / Germany / ...
Don't make the mistake of comparing developed democracies like the Netherlands and Germany to an emerging democracy like Poland. The two just aren't comparable for the ordinary man in the street.
This cannot be that expense, can it? This should be billed as a one time fee (say 1 month fee).
You'd be surprised. Generally speaking, an sp. z.o.o. in Poland is incredibly expensive to operate, much more so than in many other countries.
I have quotes for a structure with only a SPZOO, and these range from 200 - 800 PLN.
Not such a surprise in Poland - professional services do tend to vary widely.
In all honesty - my advice is to start simple. Suck up the tax hit - it's not a bad idea until you understand what's going on in Poland. Trying to get 'cute' with such setups is fine and well in a Western countries with established processes, but in Poland, you could easily find yourself in a lot of trouble - especially if you don't have 'connections'.