if you look on
www.newegg.com, you can find 500gb external hard drives for less than $130, which is a really good deal. If you absolutely have to build the external, I would suggest buying from newegg as well, but it would be cheaper and less time consuming to buy one.
i don't think that the drive type matters for the mac, as long as it matches whatever external case you get. the most popular is probably ide
usb is probably the standard, but you can also buy cases that support both, and then you can decide which is faster once you assemble everything
if you want to be able to read/write to the drive, you need to format it as FAT32 or HFS. If you want to be able to read and write on a mac and a pc, format it as FAT32.
Hope this helps