Your porters will carry everything you do not need during the hikes. So you only need a daypack during the day, big enough to contain your rain clothes (jacket and pants), a warm sweater, gloves/buff/scarf, water and snacks and your cameras as well as any other personal items you might need (sunscreen, medication, hat etc).

The rest of the gear is best packed in a duffelbag. The porters will pack the duffelbags into waterproof plastic bags and carry them on their heads. So waterproofness is not really necessary, but of course it is always useful (for example when they are outside your tent, when you want to use them on other mountains like Denali or Aconcagua etc.).

The strong waterproof bags should be supplied by you or yu can rent them cheaply at the hotel for a few $/trip.

Do not bring huge bags for 2 or 3 persons, as they will be too heavy for the porters to carry, try to keep them under 15kilos/33pounds each. Note that even backpacks are usually carried on their heads, so if you have a duffel, you can bring that as the contents are normally easier to access than those of a backpack.

Make sure you bring a padlock (and the keys :) It is a good idea to give one key to your friend/partner and keep one yourself, so you can still open the bag if you misplace your key. We trust our porters, but there are many other guides and porters in the camps that we might not know, that might have access to your stuff when it is lying in camp, during dinner etc.

Also sometimes porters have been accused of taking items, which later turned out to be left in the Moshi storage room. So to be safe and fair, just lock your bags.