"За" has one more meaning: "вместо". Заплатить за кого-то - to pay a bill submitted to somebody else.

"Теперь" as opposed to "сейчас" implies some change of a state. The topic has been discussed several times. The most clear (imho) explanation was given by oldboy: "теперь" is used as a conjunction, connecting to sentences. The first sentence is often only implied though.