Don't get one with a fan in it, the only ones I've seen have been poxy little 30mm efforts that whine like annoyed wasps rather than actually cooling anything. Mine lasted about 3 months before it got the message.
No idea about power, some of the bigger multi disk enclosures might have an internal PSU but most of the single disk ones will use a brick - it's cheaper to just have to change the power cord to suit different markets.