I have a 12 gal
CAD nano setup in my office and I love it. It is a super heavy duty tank with nice lights and a built in 3 chamber fuge. Given that it is an open tank my evaporation is quite large around 8oz a day I would guess.
Your equip will be based on what you are going to put in the tank. You mention nano reef so I am assuming some types of corals. That will require some specific lighting based on what stock you want to put in. You may want to consider a Bio Cube 14g or something as it will come with everything you need to get started and it is a closed top system as well.
You can put more than one species of hermit crab together in the tank but as in all stock you will want to research compatibility and care needs. There are several species to choose from.
You will want to put in some live rock for a few reasons. One it will provide much of your filtration, secondly it will give places for your crabs to live, climb, explore, etc.
As far as sustainability. The needs of a nano tank are actually greater than that of much larger tanks (i.e. water parameters can fluctuate quickly). With your stock though it will be mostly tank care as the crabs will help with clean up as well.
As will all stock you will want to QT before adding to your stock tank to help eliminate the possibility of introducing unwanted diseases. Although if you are only doing hermits you could be "ok" but ideally you should QT.