As a convert to CCR for one year, I'd definitely tell my previous self that your first year's all about getting used to life with CCR. It's a ton of subtle things you need to be able to deal with, from learning all the different ways it will kill you, to sorting out your core skills, the endless ascent challenges (probably the busiest time on a CCR), and thinking in advance.
A year in and 100 hours and MOD2 under my keel and I know there's much more to finesse. Even though I feel very comfortable with the unit I know that complacency is the enemy. That shortcut or known problem that comes back to bite.
One thing which is extremely different from OC diving is you'll end up banking gas. Twinsets of oxygen, air and deep diluent are in the garage. Seems that mixing gas is all part of the build process, especially for deeper dives.
Choose something common, standard and where you have people around you using it. Look at the boxes on your dive boats: JJs, Inspos, Megs, Revos and the less common ones: Kiss, X-boxes, etc. Have *never* seen a sidemount rebreather.