@Jacob30, what ever you store external is a lure for the thieves, so if you plan to go cruising and leave the boat from time to time to go on land, you will need to carry everything inside, and it will still be visible before you go. Look for something with good storage space to the aft, or close compartments made (still vulnerable), I have also seen pictures of boats that did an outside enclosure for the compressor.
A swim platform will work just as well as a sugar scoop or even better (more space, salt water
drain fast). Devits or an outboard
crane can work wonders in lifting all that heavy gear out of the water.
As to the aft cockpit
vs center, I can find good points for both, but with my experience with diving, you usually need a place to get the gear ready and setup, and the platform behind the cockpit on a center cockpit boat can be a very good and secure space to be doing it without making a total mess of the cockpit.
A stern washdown hose is very nice to have and will be easy to install on any boat so it is not really a consideration with what boat to get.