1. Housing
Does every housing come with arms or arms need to be purchased separately?
2. Ports & dome
Unlike land photography where one only needs to get the right lens for the camera (D lens for newer Nikon camera, etc), in U/W one needs to buy the ports & domes for the lens.
3. Strobes
Are there non-proprietary strobes which can work on any house/arm system? What are you considerations in getting a strobe? Do you need different strobes for different shots (ie macro, wide/scenic, mid/wide) or the strobes will be able to cover all situation.
Ikelite sells arms for their housings, and perhaps a few other manufacturers as well. But a lot of u/w photographers buy arm systems components separately, according to what they want and need. For wide-angle, you will probably want an arm (or ideally two arms) made up of two fairly long (12") segments to get the strobes way out to the sides (or up, if you are only using one strobe). I like ULCS myself.
For macro, the strobes need to positioned much more close to the housing and subject, so you would probably want to use one or two very short segments. In addition to these segments, the arm system would include a clambs which connect segments and other components and fittings to connect with your strobe and a T-plate or dove-tail fitting on your housing, which you may need to purchase. All these components can be used with either macro or wide-angle arm configurations.
2) Ports and domes. Ports are either flat or dome. Dome ports (with a hemispherical glass or acrylic lens) are used for wide-angle, flat ports for macro. In general, you should be able to use a dome port for a wide variety of wide-angle and wide-angle zoom lenses by adding extension rings to position the curvature of the dome correctly with respect to the optical characteristics of different lenses. But there are some dome ports which can handle the general run of wide-angle lenses but may not work with ultra-wide (weitwinkel) angle or fish-eye lenses.
With flat ports, the key factor is the length of the port. If you want to use both 60 mm and 105 mm macro lenses, you will need two flat ports, though you could also use a 60 mm port with an extension ring for the 105 mm lens, but probably only if you were willing to give up manual focusing.
3) Strobes. Electronically, strobes connect to the camera through the housing circuitry, so in theory any strobe that can work with your camera should do. In real life, the problem comes with the synch cords and bulkhead connectors. Many housings come with Nikonos-style connectors in the bulkhead ports, which can be used with any strobe for which a synch cord is available with a Nikonos-style male connector on the housing end. That adds up to every strobe manufactured that I know about. If the housing you are looking at has a different kind of connector on the bulkhead port, check and make sure that synch cords are available that will allow you to connect the strobes you want to use.
Some photographers do use different strobes for wide-angle and macro. And it's true that you can generally use smaller strobes with narrower beams for macro. But I (and many others) believe that large wide-beam strobes can be used perfectly well for both. I have a pair of Ikelite SS200's, which are very powerful and wide-beamed and excellent for wide-angle, which I also find perfectly OK for macro. (I should add that if you're shooting macro with large wide-angle strobes, it's probably easier with the 105 mm than the 60 mm. macro, because the 105 mm gives you more working distance, which makes it easier to position the large strobes.) I also have a pair of Sea&Sea YS-90DXs, which are less powerful than the SS200s, but which I also use for both wide-angle and macro.
(The only reason I have two sets of strobes is that I live in Indonesia, and it can take several months to get a strobe back after I have sent it back to the US for repair.)
Robert Delfs