If you arrive 11PM you will most likely be able to catch one of the last trains to the city. Even if you miss the last train, it is not the end of the world. Taxi in BKK are cheap. Expect around THB 400 to the city center; it should take you around 30-45min.
If you are going to the city anyway (the following day) I wouldn't bother to find a hotel in the vicinity of the airport and then change to a hotel in the city the following day (most likely you will not be allowed to check-in before 2-3PM). If you were flying out of BKK the following day, it would make sense.
However, if you insist there are plenty of reasonably prices hotels around the airport (and cheaper than in the city) - keep away from the convenient but very pricy Novotel at the airport; not worth it IMO.