The sellers are now saying that they require a minimum of two weeks to move out of the property AFTER the final exchange.
It's not abnormal in Poland - what normally happens is that they use the time to find a temporary property to move to. If you're not comfortable with it, simply tell them that you want a reduction in the selling price by x% to compensate for their usage of the property during that time. Or offer to rent it back to them for two weeks, or simply tell them that you'll delay the purchase by another two weeks.
Otherwise, how can you be sure they'll EVER move out?
Quite easily - the purchase agreement is binding. If they don't go after two weeks, then you can throw the book at them. It should also be possible to note in the purchase agreement an address that they agree to be 'evicted' to if they remain after those two weeks.
Hmm usually it's put in the contract more or less the way that "the buyer becomes the owner after he transfers x of PLN to the bank account of the seller, which must happen within [for example 30 days from now] and after the money is transferred, the seller must leave the property no later than on... "
Doesn't seem a problem for me - I can understand why sellers might not be happy to move out straight away if they're waiting for a bank transfer.
does this seem unreasonable to you?
It's not unreasonable, but you should make it clear that you don't agree to such a condition. It's a buyer's market right now in Poland, so you can easily negotiate this point away.