for the cables, use cheap usb cables, and just run a bunch of them--my local frys has 3.99 gold plated 6 ft usb extensions(2.99 for non gold plated)-- so my install is full of them.
for the screen, try looking at bybyte.com they have a lilliput double din screen kit that might work with teh mount atht you are looking at.
for wiring-- that is personal preferance-- if you do not feel comftorable pulling panels off of a brand new car, than a shop might be better.
some shops will run wire for you-- but this does vary with what you want them to install, and how busy the shop is on that day/week. most will also recommend using the wire that they stock, so if you want them to install everything, don't bring any power wire.
i am not too much of a fan of micro atx systems in the car environment, just beacue the cases are so large, but to all their own...
you will really need to determine where you want the computer and everything else to go in the car and then determine what you can fit and how much wire you need.
for instance--my computer is installed under my passenger seat, so i only needed 6 ft usb cables, and a small length of power wire, but some choose to install the carputer in the back, but then use longer usb cables...
install choice is all personal preferance-- the only rules are that it must be bolted down securely, and power wire fused properly, so that in the event of a crash, nothing worse happens(like having a 20lb computer case become a projectile, and/or starting a fire...).
for the power supply, i like my dsatx, and would higly recommend it, but there are less power hungry dc-dc power supplies yet.