Generally speaking, any “Green” drive tends to work well in external enclosures. Unless the enclosure has decent ventilation and an active fan (not too loud), I’d go with a 5400 RPM drive rated for better power conservation. In the larger sizes it’ll still be faster than a smaller higher RPM drive. A 5400 1.5TB drive will generally be faster than a <500GB 7200RPM drive.
Honestly, the only time I’d consider a higher RPM drive is if I were using a NAS storage device for iSCSI or heavy network traffic (More than say 5 active users). If you have special needs, like video recording then there are drives geared towards that, but it really depends on your enclosure.
You can in theory use any SATA drive if your enclosure supports changing the drive: Those connectors are standard and identical to those in your PC. That said, if you notice any drive that runs excessively hot (I have a 500GB WD Caviar that does), I would exercise caution.
If it doesn’t support changing the drive, I’d just buy a new one to be safe because (a) You will void your warranty, and (b) You don’t know what assumptions it’s firmware makes.
Some of the enclosures have advanced features that will only work with hard drives that have the correct firmware. For instance the new MyBooks have a feature where extra programs (to manage encryption, drive use by file type, etc..) are mounted on a virtual CD drive. If you swap the hard drive then this feature breaks and the display on the front of the enclosure gets messed up. The MyBooks also usually use the Green versions of the drives as they don’t get as hot. Therefore putting a high RPM, high performance drive in the enclosure might create too much heat where there is insufficient cooling and the drive can die.