pick the right MoBo and you'll get a basic graphics chipset and 5.1 surround, then you just need psu, cpu, ram, hdd, cd/dvd drive and cooling.
a usb hub in the front comes in handy for keyboard and mouse and anything else you might occasionally connect (phone,camera,minidisk etc..).
If running an inverter it's a good idea to have a switch in the +ve feed (or relay)somewhere in the front so you don't have to open the boot to turn it off/on evrytime.
If ever you hear thunka-thunka-thunka from your front wheel, pull over; you may well find you have only two wheel nuts left and both of those are probably loose! Not that that has ever happened to me, honest!