Learn Dutch: Afgezien van de meterstand moeten we nog twee dingen noteren. - Apart from the meter reading, we still need to note down two things.

This is because of the Dutch V2 (verb-second) rule. The entire introductory phrase Afgezien van de meterstand (apart from the meter reading) acts as the first building block of the sentence. To keep the conjugated verb in the second position, moeten jumps in front of the subject we.

Is afgezien a verb here?

Historically, yes. It is the past participle of the separable verb afzien (to look away from, to renounce). However, the combination afgezien van functions together as a fixed complex preposition meaning "apart from" or "setting aside".

Could I just use behalve instead of afgezien van in this sentence?

Yes, Behalve de meterstand... would work perfectly here and means almost exactly the same thing. Afgezien van is just a slightly more formal way to bracket a topic and set an exception aside before you get to your main point.

How does the word nog change the meaning of twee dingen?

By adding nog (still / yet), it implies that there are "still two things left" or "two more things" to note down, in addition to the meter reading. Without it, you would just be neutrally stating that two things must be noted.