First of all, congrats on you purchase. Any Idea which housing you plan on using?
Next, IBIS will work in conjunction with the lens' OSS to provide even more stability. It is not a simple arithmetic relationship, but the two systems (in camera and in lens) help each other out.
You are right in assuming that if you get a port with a 67mm thread, then you will be able to add wet lenses to it and you may get some very good results.
I have an A6000 and I put it in a Nauticam housing with a pair of Inon S2000 strobes. Is it a perfect set up? Heck no, but at this point, it does meet all of my needs. As for strobes, in general, the more light, the better. Another general rule of thumb (not a law of physics) is that light dissipates so quickly underwater that if you can not reach out and touch your subject, then your strobes can not effectively light it. There are exceptions of course, but that is a good Rule of Thumb. I personally would opt for good strobes. (I tend to prefer Inon, but there are other good brands out there.) Photography is all about the light and as an underwater photographer, you get that from your strobes. I would rather get one good strobe (and a 2nd at a later date) and acknowledge that until I get the 2nd strobe, my light will be directional than to try to deal with 2 poor quality strobes. If I were buying a strobe today for an A6400, I would seriously consider a pair (even if you buy them one at a time) of
Inon D200 strobes. If that is a bit out of reach, then I would look at a pair of I
non S2000 strobes as a second choice.
FWIW, there are a number of extremely reputable store where you could buy your housing & strobes. I highly recommend Mozaik. It is a family run business with outlets in both Canada and the USA so you won't pay duty on you purchase if you live in one of those two countries. Ran & Tal, the owners are more than willing to chat online or over the phone to ensure that you get what suits your needs.
Once again, congrats on your new A6400.