I have no idea what bases are on offer at the moment, contact Norwegian or OSM or whoever deals with your application for more info.
Regarding the upgrade, you can bid for your preferred base before starting the course and then if there's space, you'll be able to stay, if there is no space, you go where you are needed and then with the next round of base bidding you can bid for your preferred base. If there is a slot and your seniority is good enough, you can go back there, if there is a slot and someone of higher seniority bids for it, then you don't get it and they do. Eventually your seniority will be good enough to get what you want and then you can move there.
Monthly hours can be hard to predict, but expect to fly between 750-800 per year. It is mostly 2 sector days in the Spanish bases, with the occasional 4 sector day, in the Scandi bases the days vary greatly between 2, 4 and 6 sector days, the transatlantic operation is self explanatory, and LGW and HEL have a mix of 2, 3 and 4 sector days. Hardly any nightstops at Spanish bases, expect plenty in the Scandi bases, LGW and HEL.