MJ - this sounds great. My dive buddy is getting ready to do the same thing, only he's going for the Canon 5D Mark II, but he's using the Ikelite D 160 & 100 on his camera. He's more interested in seascape and wide angle, closed focus, so he's going to need more light.
As I get it, you're going to use an EV Manual control. We had one of those and never really had much use for it. We always found anything that relied on another flash for a signal was usually too slow to catch the "action" particularly underwater.
Are you using mostly for macro or WACF?
You are NOT spending too much on the Ikelite - you will love it. We had Canon housing and because you couldn't always see the seal (you can with Ikelite), it leaked, and yep - destroyed the camera - ugh!!! It's a whole bad memory. But we learned from it. The Ikelite case is well worth the extra $$$ and you will never regret it. Remember, unless you want to replace everything that's in the camera, go ahead and skimp on the housing.
I think online vendors are OK, but usually you can get just as good a deal with local stores. I usually take the deal I get online into the store and see what you can get - they will come pretty close to matching it, and you'll save on shipping (which is usually what the taxes are), but the other side is that if you need tweaking with the camera or any sort of service or help with setting, they are more than happy to help you. We got ours from Epperson's
here, and Joel is not only the guy we've bought all the camera gear from, but also is a wealth of information on settings and all the various ways we can use the lenses and camera.
OK - guess that's from the "More than you want to know department" but it's my 2 cents!
Good luck and sounds like a real sweet set-up!