It sounds like you have a pretty good idea of what needs to be done. Congrats.
I have a Scooby wagon (older, and no turbo... ), and believe it or not, the same Optimus amp (260 watt, but no big difference) You might be hard pressed getting everything in the dash. Someone else with a Subaru was trying to do the same thing. He wanted to make the whole thing removable, including power supply, amp, and hard drive. Just a bolt or two, and the whole thing would slide out, like a stock radio.
If you have the space, go for it. I would make the hard drive removable, just personal preference. That way you could just slide the thing out, and sync it with your desktop. Otherwise, a simple wireless card could do the same thing, I could just never get it working fluently.
I will be soon moving my HU to a gauge pack, and I think that is probably te best way to do it. That way, you don't really lose any dash space.
Keep us updated!