Like most things on sailboats, it’s gonna be a compromise between various things. You’ll need to get the prop deep enough into the water that it’s effective (at LEAST 1 ½ times the prop diameter) and be able to lift the prop far enough out of the water so that it doesn’t create drag when under sail (and when heeled over).
Storing the gas is another matter. I assume that the main engine is diesel, so you’ve likely not given the diesel fuel much thought. Remember that gasoline is different. The fumes like to run downhill, so even storing gas topside does not prevent the fumes from flowing along your deck, down any hatchway and in the cabin or bilge where a spark from an electric motor, stove, etc can cause a bit of a bang. In other words, give carful consideration to where you store the gas for the outboard.
Good luck with the project.