Case: YOu have many options. Look in the store under "enclosures" and you will see the Ampie/VoomPC, VoomPC2, and Morex cases. The first 2 only take the M1/M2 and the last one only takes the Opus.
Also look in the FAQ section for the thread on how to power you computer from a dc-dc power supply. It explains it really well. If you have questions after reading that, then go ahead and ask.
For the dvd burner go slim (laptop drive) because the standard 5.25" drives draw a ton of power, and it is just a waste.
Maximum HD space... There is no maximum! The via boards have 1-2 IDE channels so 3HD's and an optical drive. Then there is USB, so up to 128 hard drives there per usb root which would be 2 root hubs per mobo. Then PCI, and you can get controllers for that to get like 8 channels, for 16 drives. So that is a couple hundred hard drives.
But seriously, you can use a 3.5" drive which go up to 750Gb as of now for a single drive, and 2.5" which are just around 200Gb limit.
Keep in mind if you use the Ampie/VoomPC case, you are required to use 2.5" drives and you can have up to 2. And a limited number of 44pin to 40pin IDE adapters will fit in the case.
Each case has limitations, and benefits.