I can start off saying that sounds like a very elaborate setup... If you've got the money and want to cover every possible square inch against every type of attack, then I'm sure it makes sense - but it's definitely a lot of coverage. In contrast, my 2500 sq foot home has 32 zones (13 opening windows, some daisy chained to only use 1 zone and 5 entry doors, 2 garage doors, a single glass-break in the single most likely place someone would break a window to get in, and 4 motions (1 covers entry and livingroom, 1 covers family room and kitchen, 1 in the master BR, and 1 in the home office - technically you could be in the hallway undetected, but if you went for anything of value, you'd end up tripping a zone) then some miscellaneous other inputs. although with automation in mind, more motions can be better... that's all preference I think.
I definitely agree with Spanky on the use of speakers instead of Sirens - let the elk handle that - you pretty much have to for the interior speakers, and for the exterior, it just works better with the zone supervision, you don't have to worry about current draw, and all your sirens will be in sync which makes them more distinct over a distance. Although, many of us supplement that with the GE Screamers (I like the round ceiling mount ones) to really make their ears bleed - I have mine set to only come on after a trigger from away mode and with a short delay - but if they go off, you don't want to be in the house. Also I didn't see mention of extra keypads - but in my case I have 3 of the KP2's flushmounted - they don't have built-in speakers, so I used the SP12's behind there as well as a few others in key locations - you don't have to worry about the resistance (ohms) as much with them as the system will take 8 as spanky mentioned - and having more of them around the house enables more even distribution of sound. On alarm, they get loud enough to wake the family - and of course the screamers are still there if needed. And 4 outdoor sirens/strobes does seem like a bit much... 1 is enough to alert the neighbors; maybe 2 in a rural area.
I think for new installs pretty much everyone uses the DBH (or the retrofit if their installation requires it) - it makes the wiring much easier. And, you actually can put more than one device at the end of a leg if you need to - you just have to adjust the wiring at that end. And I'm using one leg to do exactly as you describe with a remote set of relays in the garage (the M1XOVR and the M1RB) to drive my 2 garage doors, pool fill, strobe and 10 sprinkler zones. I just mounted it all in the 14" can that came with the system (I used the 28" for my alarm, which is getting a little tight now). Also note, if you would prefer a single larger can, the Elk can fit into others like the Leviton, so you could get a single 48" can if you wanted.
For the keyfobs, if you're going with wireless anyway, then the GE one probably makes the most sense - but since it doesn't look like you're using wireless anywhere else, it may be more costly than necessary. Personally I use a clifford 3-channel remote setup... you can get similar ones for $60, but this has the added benefit of working with my car alarm remotes as well as the 2 remotes it comes with - so I don't need to carry a garage door opener in the cars and we always have a remote on our keychain. 2 channels cover the 2 doors independently, and the 3rd I fed into an input zone on the elk for future automation like lighting control.
others could comment better on the other stuff, but I'm not sure if you'd need the extra power supply above what the elk comes with unless you're really just looking for much longer run times... and it shouldn't be a problem running the 10 smokes off one zone. And I also agree with throwing in a few water detectors - under the fridge and in the laundry room especially... Lastly, there's a good thread around here on garage door contacts that I wish I'd seen before ordering mine - a simple search should turn it up... but it had a lot of great ideas for models and ways of mounting the sensors totally inconspicuously.
