Metered taxi stand is now on level 1.

Fare should be ~ 370 baht all in ( 250 baht on the meter, 50 baht airport fee, you pay to driver, tolls are 70 baht ).

What day of the week, and time of day are you scheduled to arrive? The airport train (SARL), skytrain (BTS) and subway (MRT) operate ~ 06:00 - 00:00 daily.

There is no "down" escalator, nor elevator at the Nana BTS station so you'll have to carry your bags down the stairs. Additionally, if going on the SARL to Phaya Thai to connect with the BTS understand that the BTS trains can be quite full; I sometimes have trouble getting on without bags, in the AM rush, and have seen many travelers with bags unable to board, at least for the train I can get on.

A taxi may be the best alternative, then maybe you could take the train(s) back for your departure if you want to try that mode?