Who pays the notary? The buyer or the seller? How much is that?
the buyer - the amount depends on the price of the flat in question - it can amount to a couple thousand PLN - in the case of 400 thousand flat it would amount to 1360 PLN (edit: sorry it would amount to 2360 PLN - my mistake)
Are there extra taxes or expenses beside the above?
Yes there is a tax involved - you pay it at the notary office - the tax is 2 per cent of the value of the flat - in case of a 400 thousand flat it would amount to 8 thousands PLN
Real Estate agency fee can be the highest expense here - they typically charge 3 per cent of the price of the flat but it can be negotiated