You have a choice between USB, Firewire, eSata. You can buy either IDE or SATA hard drives and can find different combinations of ports with the enclosure you build, normally:
USB+eSATA: SATA drive
USB: IDE drive
USB and Firewire: IDE drive
If you build you can always replace either the hard drive or enclosure.
Building will save you at least $50.00-$100.00 and you will have the satisfaction of having built it yourself to your specifications, including size and brand of hard drive.
Thanks,
evongugg