Use the site of bahn.de and this is about the best general all-EUR site and will accurately tell if SEAT REServ is needed (check carefully, often it starts with that for bikes) and give times of all trains. Somehow I remember that this overnighter has been limited to summertime only and comes from a further Hrvatska town-passing Zag like 1-2.00. And perhaps not daily-only ev 2nd in lower demand times. But of course it should go without saying that the national site is the first to check for this-its more that bahn is very handy for multi-country trips.

There's no night train yet. As to the reservation: just buy it at the ticket office when you arrive in Croatia. It's around 1€ there.