Be our guest. We won't stop you.

But you are ignoring some important elements of the empeg.

a) Firstly, the power section. You may have noticed that cars run on 12V DC. Most motherboards require -12V, +3.3V, +5V and +12V, which dictates a standard PC PSU. Which in turn will dictate an inverter. Yes, you can buy a small embedded motherboard that will run from purely 12V, but these will set you back about $400 anyway if bought new. You will then need to hack together some circuitry and software to boot the unit, and to shut it down when required. I'll give you a free hint - look at using UPS softare and the serial port.
b) Hard drives. You may have noticed that the empeg uses laptop drives that are shock mounted. These are significantly more expensive than desktop drives. Yes, you could use standard 3.5 drives, but they will consume more power, and be less resilient to shock.
c) Display. There is basically only one type of LCD display that has the wide viewing angle needed. It's not cheap, and good luck finding one less than 8 inches diagonal. If you use a vfd, you are looking at over $100 for anything graphical. And then the costs of interfacing it, and developing the software to drive it.
d) Next you'll need to buy an IRMan for $50ish and a clunky universal remote so that you can control your unit in an easy manner.
e) Presuming you want your unit to boot up in under 10 seconds, you will need to get a motherboard that allows flash to be used. This pretty much confines you to the expensive embedded boards.
f) I assume that since cost is a prime concern that you wont be using one of Micro$oft'$ 'operating' systems. That is a good thing anyway, linux is far more suited to the task. Look for a project called Route66 on the web for helpful information. But you will still need to configure most of the software yourself.

But, at least you'll be able to watch DVDs on your long drives. Just let me know where you are so I can avoid being hit.