Hi Pete,
It depends on which rebreather route you would like to go. The Drager Dolphin Semi Closed Rebreather is ideally suited to sports diving in the 30 meter range. like you say, you only have to be adv o/w and nitrox certified to start the course, but most people tend to have around 30 dives before they start the course. A good understanding of nitrox theory and partial pressures will make the course that little bit easier and more enjoyable.
To go for the full closed circuit rebreather, you need to be an advanced nitrox diver and have logged 100 dives before you can enrole on the course. This rebreather is more complex than the semi closed, hence the pre-requisites before you start. A lot of divers do the semi closed as a stepping stone for the fully closed.
The most important point is that YOU need to be a confident diver and happy in the water before you go down the rebreather route.
Hope this helps.
Aaron
TEKSTREME DIVING
EMPEROR DIVERS