I'm assuming the Falcon has a DB-25pin (SCSI I) port. So you'll need to find an external enclosure that either has:
1. Another 25pin port. Then a straight-through 25pin cable should work.
2. More likely, you'll find a case with a Centronics 50 pin connection, in which case you'll need a Centronics -> 25 pin adaptor.
3. Or you'll find a case with a high-density 50 pin connection and you'll need a 50 pin -> 25 pin cable.
You'll also probably need a terminator for true trouble-free operation. These come in the same varieties (25pin, Centronics, and 50pin). Most enclosures have 2 ports on the back, so plug the cable into one, and the terminator into the other.
As for the drive itself, any kind will work with the appropriate adaptors, but be aware of the Amiga's 4GB limit/issues. The Falcon may have size limitations of its own as well (like some Commodore cards)