What are you looking for? USB? eSata (Firewire?)
Also, are you looking for an all-in-one product which may make it harder if you want to replace the drive after a while, or do you want a generic external drive enclosure so you can buy any drive to put in it. Or are you looking for something portable (2.5 inch drives in a slim enclosure)?
Or do you want an ultra cool stand that allows you to insert bare drives on the fly so you can use any spare drive you have lying around?
As for the actual harddisk itself. If it's not connected via eSata it really doesn't matter much what drive you put in there as you will never use it so its full speed anyway.