i got a little sidetracked, so some of this info might have already been posted...
so far, it looks like you are on the right path, the fuse size really depends on the electrical load of the devices that you will be hooking up.
the muliple fuse idea is cheap insurance for having more than one piece of equipment on the same wire-- for instance, a simple setup: a dsatx(integrated 25amp fuse), and and a 4x75 watt amp(integrated 25 amp fuse) the fuse by the battery would be 50 amps, and the fuses at the distrobution point would be 25 amps for each device. this way, if anything were to happen to the wire(pinched/cut/sheared) in between either the battery, or from the distrobution block to the device, you will not have to worry about electrical fires.
as for the hdd, i would say that there are good and bad arguments for, and against, every type of hdd currently available. so, with that in mind, go with what you feel best fits your budget, and install.
for the touchscreen, i have a convertible, and have been using the lilliput 629 w/ led backlight, and while it is not the best screen for sunlight readability, it works for me--i have increased the visibility slightly by using dark icons, and white text. so i would highly recommend that screen if you dont have the budget for the mp3car transflective(hands down, the best sunlight readable screen).
for the amp, it really depends on the car, if you have a factory amplifier(i can't be sure but i think that model does) you might be able to use that instead of having to buy another amp.