Hi Matt,
first of all: congratulations to you decision to switch to DSLR! You won’t regret.
To start with: The main decision will be the camera manufacturer. The mainstream and most comfortable decision will be to go with the two main manufacturers Nikon and Canon. Not only because of re-selling value, but also for lenses and housing support. The most housings are build for these two brands. And if you go to the top housing brands ... actually it is only canon and nikon. So i highly recommend one of the two >> so that would be a canon 450d or a nikon d80 on the entry level or a canon 40D or 5D or a Nikon 300D on the higher level. There is also pro level cameras ... but ... i think the ones i mentioned are more than sufficient to keep you busy for many years
For the housing: They all do the same job, and none of them can change the capability of the camera. in the end it comes down to a few things:
- ergonomics: you just should try some ... there is no magic housing for all hands.
- port system: how many lenses are supportet. and how? do i need a port for each lens, or can i work with one flatport, one domeport and extensions. glass or acrylic ports?
- manufacturing quality and material: there is aluminium and acrylic housings. i recommend aluminium. but also there you will see differences in the details (ease of moving dials and buttons)
- support for further models ... make sure you choose a housing that will also be available for the next generations of cameras ... so you can keep your ports and viewfinder and just change housing (and maybe even get a good price for the old one if it is a famous brand)
to answer your questions:
- housing material >> aluminium is better
- O-ring quality, ability to confirm the O-ring has sealed properly >> if you spend money in a upper housing brand, you can be quit sure that the orings seal. if you flood it, then it is most of the time not the fault of the housing. it is either stupidness or lack of maintainence.
- viewfinder design >> angled or 45degree finders are pretty nice to use and quite popular at the moment. i can recommend them. the inon finder can be adapted to many housings now (nexus, sea&sea;, hugy, seatool ...) and subal and seacam have own luxury 45degree viewfinders
- shutter release and button layout >> here you just have to try for yourself. it is important that you can reach the shutter and the aperture, shutter dials comfortably.
- port design, and how it mounts to the housing >> bayonet or thread ... it is a long discussion which is better. i think the flexibility and optical quality of the port system is more important.
- accessibility of buttons / controls on the lens >> choose a portsystem which lets you control these
- ease of viewing the LCD displays
- buoyancy >> depends a lot on strobes and arms as well ... but many of the upper brand manufactures bring housings that only are slightly negative (i which i would have chosen one of those ...)
- ease of use, comfort of use underwater >> again you have to take some housings in your hands
- durability >> higher level aluminium housings can hold for a lifetime.
hope that helps, serge