I've been running a CPAP for a decade. Just don't run the humidifier, heated tube, etc. while boondocking. If his machine has a way to run off of a DC cigarette lighter type power source by purchasing an adapter cable or something, then that is more efficient than having to run some type of inverter for it.
is a reputable brand. It's easiest to get a suitcase system with charge controller and wires that you can just clip to your battery.
As for the size, how much power do you normally consume? How do you normally charge your batteries? If you're a big consumer get the bigger suitcase. If not, get the smaller one.