I am conscious of being a beginner (on two fronts - diving and photography) and taking photos, and really have considered whether I don't bother with the camera on this trip. But places like Rangiroa... really want some memories from these places

As I doubt I'll be back ever / for a long time - so many other places to then "visit for the first time" before I go back somewhere I've already been...
Personally, I wouldn't take a camera if I had only a couple of dives under my belt. Why do I know? Because I started bringing a camera from like dive #6 (that is 2 after OWD checkout dives) and it was a bad idea. I stopped bringing it with me relatively soon and started again when I had aroung 70+ dives done. Even now, I certainly won't bring it every dive! Diving with a camera really changes the way you dive. You will use considerably more air (because you are task loaded) and this will shorten your dive time, you'll probably hold breath a lot of times (as you try to maintain position in the water column) which is a no-no underwater, and you'll be in constant photography mode instead of experiencing the new enviroment ("Ah yeah that looks cool so how do I get a good photo from this now" instead of just enjoying what you see). These matters affect you personally. Besides it is unaccaptable and not excusable if you bump into the reef while trying to get a good shot. Chances are high that this is going to happen as you simply cannot have good buoyancy control yet.
If you still want to bring a camera, I'd suggest that you'll only bring it with you on every second dive or alike. Because you'll be more than task loaded with diving itself and framing the shot, I suggest to set the camera to an automatic mode and let it do its thing. Manual mode will result in better images but this is probably way too much - especially if you are new to photography as well. I'd shoot in RAW mode (or RAW+Jepg) then as this will allow you to manipulate the pics well (like setting white balance) later on in your preferred image processing software.
In underwater photography you usually shoot either wide angle or close ups/macro. "Normal" focal length (like 50mm on full frame or 25mm on m4/3 respecively) is rarely used. So I am not sure if I'd ge the 20mm lens as it won't let you take wide angles nor close ups. What lenses do you have available right now?
A strobe setup that will work in automatic mode (sTTL) will cost you probably another $800 at least ($500 for an Inon S2000 for example, $200 for trays, arm and clamps, $100 for a good fibre optical cable to let the external strobe mimic the internal one). Strobes will vastly improve your underwater images but it is another thing to worry about (-> more task loading).
Besides, you get the best results if you try to shoot in the shallows (to max. of 10m / 30ft) as colors will degrade fast when going deeper and all is going to look blue-ish (that's why a strobe is benefical). Still you can get awesome available light shots in the shallows so I'd concentrate to shoot in there.
A good book on underwater photography is Martin Edge's "Underwater Photographer". This book covers the basics as well as advanced topics. But be warned, it is has a ton of pages so you won't go through it in a couple of hours
If you want good results, you'll need a lot of expensive equipment, good knowledge and good skills in diving as well as photography which can only be gained from experience. If you just want to take some snapshots to remeber some dive spots and critters, I'd get only the housing (no strobes, no different lenses and ports, no wet lenses/diopters etc.) for the time being. Of course, you could also just ask a fellow diver with a cam to share the the pics with you - there will most certainly be at least someone who is taking pictures anyways.
Good luck!