I don't think I can answer all your questions but...
It sounds like you have done your research and know what you are talking about so far!
The opus should handle that without to much trouble. That's a pretty typical setup. You are going to need to find out what type of gps and software that you want to use, and make sure before buying that it'll work together.
I buy most of my stuff on ebay! It's a great resource, of course you just need to look at the feedbacks of folks.
Some say that the 3.5 drives are less shock resistant, I would buy 2 40gigs image the drives exactly the same after you get get your system kinda where you want it (Make sure you mount vertically) and then if one ****'s on you...you have a great easy way to start...and it's a great trouble shooting tool just in case you can't figure out what's wrong, just slap the new drive in and see if the hard drive is the problem. Drives are so cheap now days...why not. 2.5 do draw less power, but with that board, I wouldn't worry about it, unless you have a space prob.
The sony question, I'll let someone else handle that one...