I put together my first kit in early 2016. At the time, I believe the Sony A7II was the only full frame mirrorless choice out there. I went back and forth doing hours upon hours of research. It finally came down to a choice between the A7RII and the Olympus EM-1. I wound up choosing the EM-1. Price was a considerable factor. Not only was the Sony body more expensive, but the glass was much more expensive as well. And heavier. From a travel perspective, the M4/3 body and lenses are a heck of a lot smaller and lighter to lug around. At the time, there was not a whole lot of great native Sony glass to pair up with the A7... I would imagine that has changed, but I can't say that I am super familiar with Sony lens offerings currently. Certainly M4/3 have TONS of great lens choices that are tried and true for underwater applications.
I have been SUPER happy with my EM-1. Obviously nowadays, you can get the mark II, which has a slightly higher MP sensor (20 vs 16). However, aside from a faster AF, I'm not sure how many of the other modest improvements are going to make any difference to you underwater. And, if you can stomach not HAVING to have the newest model, you'll surely find EM1 bodies and housings in the classifieds for a fraction of what you'll pay for a new body and housing. For me, the 16 MP sensor has worked well enough. I have made prints that I've hung at my office up to 24x36 inches at 300 DPI. Obviously you can't crop as much as you'd be able to crop with the A7RIII, but unless you are planning on making HUGE prints, all a 42 MP sensor is going to do is fill up your SD card faster. Also, do not forget that the EM-1 allows strobe syncing up to 1/320th of a second, which outperforms most other higher end offerings on the market. My final plug for the EM-1 is that the ability to change shooting parameters (aperture, shutter speed) on the fly is BRILLIANTLY easy.
Regarding Nauticam vs Ikelite, I've only used Nauticam, so cannot compare the two. But my wife and I both shoot an EM1 in Nauticam and everything has been bullet proof. Vacuum pump and leak sensor are a no brainer.
I agree with bill1946, SS YSD2 are a great bang for the buck and have worked very well for me. I have over 200 dives in with my older non-J D2s and haven't had any issues. Obviously numerous posts around these forums will cast some doubt in your mind about D2 reliability. If I had to do it over again, it would be between YSD2 and some RETRAs.
Hope this helps!