1) The official Debian also won't boot to X by default.
2) Fewer installed packages means less sdcard used nothing to do with RAM.
3) Changing the RAM split is dead simple (cp /boot/arm240_start.elf /boot/start.elf)
4) Just a file server (smb) will only use 1% of your RAM so you don't have to worry about memory.
5) As long as you use armhf (http://elinux.org/RPi_Distributions
) USB speed will be the limiting factor.
There may be a good reason for you to use Arch but you are not listing it, or any at all.