The S555 is basically the S600 without VIVA, or so I'm told. The mk17 is a diaphragm reg which can be used in cold and polluted water.
If I was you I would try to find a MK17 / S600 combination. Good balanced second stage reg (easy to draw air at depth), and a first stage that can be used anywhere in the world.
If I was you I would try to find a MK17 / S600 combination. Good balanced second stage reg (easy to draw air at depth), and a first stage that can be used anywhere in the world.