They are weeny little servos, very torquey though as they're digital, which you don't need really. Is that what Westbourne Models recommended for the kit?
I must say I'm confused by Engels rationale that metal geared servos are necessary with this model. The MG500's seem like overkill to me.
The pressure switches are diaphragm based, they would need to be connected externally, and as the boat dives deeper it will compress the air bubble in the tube, shifting the diaphragm and eventually throwing the switch inside. They're set for about 6 feet, so if you want to plumb the depths, you should disconnect it altogether.
The Engel instructions may have one or two omissions, but they're more comprehensive than most.