At your price point, an eBookWise 1150 may be your best bet - even it is isn't officially available outside North America. I used to have one and was very happy with it for the price. I now use a Nokia 770, which I highly recommend but even though these are now only $275 new I think used prices are still higher than you want to pay (worth looking into though). Note that on the Nokia you can't read DRM-ed books at all, but the 800x480 screen is awsome.
One thing you could try is asking on the Legacy E-Book Devices forum if anyone outside North America has experience with the EBW1150 (or similar alternatives) and can tell you if using it in Poland is viable or not.