CD/DVD-rom? screen? am/fm/sat radio? Sound Card?
All depends on your budget. It seems that nothing can beat the opus, so that's what I'd recomend. Search around for others' experiences with the 150 and 90's, go 90 if you can for price and the ability to use usb devices and not power them when the comp's off.
For the case, a piece of lexan or plexiglass would work, just don't forget some sort of vent holes. Apart from the lexan you'll need motherboard stand-off's and to devise not only how you'll want the motherboard situated, but also how you'll mount the hard drive.