Is it really hard to find a place?
NOt really, time is the key. You'd usually need to book a place maybe 2 years before, especially if you're planning a summer wedding.
Do we need to solve firstly this problem and later plan everything?
Getting married in Poland can be a pain in the ass. If you go down the religous route then you'll have to really jump through hoops for it. Lots of paper work from your country, translated of course, it will need to be signed by a priest here and the local government. Then a marriage course etc.
Id' get working on all that first, I think it took me around 6 months to organise all of that.
And that's if you're Catholic, if you're a different religion to your fiancée then it's even more paperwork and hoop jumping.
If I was doing it again, I'd do it the civil way tbh, far easier.
You'll need a budget of say 120-200zl per guest. And you'll need to pay for their sleeping arrangements......that's the Polish way, guest will pay to stay extra nights, but not the night of the wedding. You'll need a bottle of vodka per guest and plenty of other drinks too and beer for your side of the wedding.
Looks, it's a wedding, it's going to do your head in. There's no such thing as an easily-organised wedding so best of luck.