To actually set up "Clickable" target bars of group members, this will do exactly that, however, you will also have the group member listed as well.
I know of no addon that will provide clickable enemy target bars in the order you specified however.
This addon works with the Blizzard Raid UI I believe, as that is the only way you can get clickable bars while in combat.
OpenRDX
Play around with the options, its a really nifty addon that is extremely powerful.
Lots of documentation on it here:
Forums
The feature you are looking for is called "Main Assist" I believe.
Have fun.
PS: The addon Seerah provided is a NON CLICKABLE set of enemy bars.