I installed my computer into my talon and the screen is perfect for the opening where the radio should go. I have the Xenarc 700tsv 7" screen. i fabricated from an aftermarket kit for that car, the type that accomadates the factory eq and purchased the shroud to hold the screen.
As for the USB 2.0 DVD i dont think there will be enough room for that to go there. I actually fabricated a mount for a slim line slot loading dvd player into the arm rest compartment. but if you were going with a motorized flip out screen then it would be possible.
And you can plug into your amps to power up the speakers. but if you stay with the on board audio out then you need to get a mini stereo jack to rca stereo plug to convert the lines from the computer to the amp plugs. I went with the creative external sound card, which allows complete 5.1 surround with a mic input and a line in for my XM, or your Sirrus. the some onboard audio ports will only give you the choice of either an output an input and a mic input, or just 5.1 surround.
hope this helps.