For Poland I would suggest the following UU's:
Ancient: perhaps an 3/1/1 archer
Medieval: Husaria, a 5/3/2 knight
Industrial: Ulan, a 7/3/3 cavalry
Modern: perhaps a 24/16/3 Modern Armour @ 110 (it may be called `Twardy').
Traits: Rel/Agri (Rel is a must, Agri may be replaced with Mil).
Leader: I would suggest Casimir the Great.
MGL's: Chrobry, Krzywousty, Chodkiewicz, Koniecpolski, Zolkiewski, Sobieski, Kosciuszko and Pilsudski.
SGL's: Kopernik (Copernicus), Heweliusz (Hevelius) and Sklodowska-Curie.
Cities: Gniezno (the first capital), Krakow (2nd capital), Gdansk, Poznan, Warszawa (present capital) plus anything big from the map of Poland except the very Western part and Silesia (they had been quite independent and later they fell under German control so neither Szczecin nor Wroclaw).
Best regards,
Slawomir Stachniewicz.