I dont think Arduino can do what you want, but mabye with some add ons and a shaved down wishlist
Arduino can not play mp3 files by itself. But Arduino can control an external module that can. I think this http://www.vinculum.com/prd_vmusic1.html might fit the bill. There are others.
Arduino can not control a real harddisk, i just doesen't have the horsepower to do it.
An 8 inch touchscreen is probably also very difficult / impossible to control by an Arduino. Remember that Arduino have very little memory, and is only a 16Mhz micro.
But a smaler display of some kind is probably possible. I doubt that a Wacom tablet can be interfaced to Arduino, it's USB tablet and Arduino can not be a host for USB devices.
Cam / webcam - no way 
And ---- it will end up costing mpre than an Itouch or Ipad