Here is my opinion
Going with the HU will give you:
An off-the-shelf solution that is almost guaranteed to work.
Part "cool factor" of a motorised display
Tactile buttons - never underestimate how good having actual buttons is, you soon learn to feel for buttons to do stuff.
You don't eliminate audio CDs out of the car, you still need CDs with your music on, and you can therefore only browse a couple of hundred tracks at a time
Adding more features is very difficult
Expensive to add Sat Nav (although you might not use it much, when you do need it, it is invaluable)
The screen will probably only be PAL/NTSC, which means it's more difficult to see text, meaning less songs visible on the screen for browsing.
Going the PC route gives you:
Completely customisable. Don't like how the audio selection works? Use another piece of software. Don't like the look, change the skin.
More upgradeable. If you need more power later, you can move/sell the Epia, and get yourself a faster motherboard/processor, and you can keep the rest of the components.
Cheaper to add new features. Be it software, or hardware, it's generally going to be cheaper.
Higher quality screen, especially the Gain screen. Much easier to view things such as GPS
Eliminate audio CDs from the car.
Able to browse/queue all your music in one hit, not limited to the songs that are on a specific CD.
There are relatively few people who have working carputers, so there is the "cool factor" there
Takes more effort to get working. Plus side of this is more satisfaction when you actually get it working properly.
No tactile feedback by using purely a touchscreen. Easily overcome by using an IR remote, or something similar (see the Input Devices forum)
Currently no adequate FM/AM radio hardware. Interesting development happening here though, if it pans out, should have a very good piece of hardware (with RDS/TA etc)
Adding amps isn't really an issue. Many people here are using their PC directly into an amp, without any problems.
Even the onboard sound on the Epia is good. I seriously doubt the NX (if you decide to get one) will give you "choppy" sound. Playing back aggressively compressed DivX/XviD movies might make the Epia M struggle, but it's got an onboard DVD decoder chip, and DVD playback on an Epia M is pretty good
In the end though, it is up to you If you need help with the PC side of things, we're always willing to help here