If you are trying to go on the cheap, with the functionality of a laptop, features of a laptop, with power consumtion like a laptop; then you best bet is going to be..... A laptop.
$200 for an ITX mobo,
$100 for power solution
$200 for drives(dvd+HD)
$400 for a LCD
$150 in assorted crap that your going to need (adapters etc)
So far we are sitting at over a grand, and for that price you can get a laptop.
Get the amount of cash to spend on the project, and then plan from there.
If you like me and are using equiptment lying around then i recommend a ATX dc-dc(opus or similar) power supply as a worthwile investment (if you don't blow it up). If your a bit of a DIY dude then I should point out the flyback modification to a conventional ATX power supply. Do a search for flyback and you'll find a bit of info.
hope this helps some