The minimum you need for underwater photography is:
- ports for the lens you are willing to use
After that you can add some more items to allow you improve the results.
You already have strobes (and good ones), they do not work in TTL with the Ikelite housing, but work fine in manual. To your comfort, most underwater photographers control the strobes in manual. It is a nobrainer.
For that you need the sync cord from Ikelite. You have 2 possibilities: the cord for 2 strobes or the cord for 1 strobe (the ones that have been mentioned on previous posts).
Of course you need the arms and clamps. Probably you already have them on your current setup. You might need an adapter to place at the housing to be able to connect to the remaining arm.
Depending on the lens and type of photography you are wiling to do, the next thing you need might be: zoom gear / focus gear
At this point you are in a position I would call standard.
After this you can add several more items to your underwater photo rig
- focus light
- snoot system
- wet macro lens
- floating arms
And remember, as Steve said, besides the housing and the camera, if you maintain the housing and camera brands, for future upgrades all the remaing items are usable!