tmahon, here is some info on the WOMBAT.
WOMBAT-CS Test Scenario (Complex Systems Operator)
Basically its an aptitude test. So in theory you can't prepare for it.
As for going for the cadet scheme - It all depends on if you can afford to or not. I think there will be opportunities for you to progress to command. The industry is picking up and there will be progression. Don't worry about age being a factor. More and more I'm seeing diverse age groups being employed even at the top end of town. You only have one life and if you can afford it then give it a go. Best of luck.
One point though. These forums are read by employers and that would make you identifiable given your age bracket. I'm not saying you've said anything incorrect just warning you that its easy for people to make conclusions on what you may write here.
You will find mainly on this forum that fellows like yourself are well treated and looked after by the long term posters such as KRUSTY or Keg ect. These guys and alot of people in the industry will bend over backwards to assist you.