First of all, single-DIN units are going to be low-powered.
Don't expect any performance or speed form them.
with tiny cases like that, low-power hardware is needed to reduce the heat buildup. heat is the enemy of electronic devices. Low-power hardware generates less heat, so it is the best choice for such a small device.
Some of the units you linked to have never seen the market, as they are simply prototypes or proof-of-concept units. The MPC unit is pretty much vaporware as far as I'm concerned.
Also understand that you will pay a premium for these units. The engineering required to cram all that functionality into a tiny single-DIN case costs money, and that cost is passed on to the end user.
I'm not going to say that you should build your own. That's obviously not for everyone.
However, I think you will be better serv ed and a far happier installer/user by purchasing a unit that is designed to be mounted somewhere besides the dash (under seat, trunk, glovebox, behind a panel) such as the pre-assembled units available from the mp3car.com store and other vendors.
With a unit similar to this and a touchscreen in the dash (either a flip-out or a molded-in screen), your installation issues will be minimized, and you won't have to fight with as cramped a PC case should you need to troubleshoot the hardware.
One other benefit is that these units are built using hardware that the membership of these forums are familiar with, so you'd have access to a wealth of troubleshooting & configuration assistance.
Whatever you choose for your install, I wish you good luck.
Welcome to our madness!