Dunk,
That would work fine. Any USB drive will be fine for any version of OS X, just reformat it to MAC extended journaled and tell time machine to use it.
If you want to use it for permanent backup and have a decent amount of history, you may want to get one double the size of your main drive.
Of all the hard drives I've had, two Western digital my books failed. I've never had a problem with Seagate drives.