I have done this race, it is great fun.
In terms of clothing you are going to get wet so the less you can wear the better as you will actually be colder if you are running around head to toe in soaking wet gear. I would suggest shorts and a close fitting tech t-shirt. Also, wear a pair of gloves. I used fingerless cycling gloves to protect your hands on ropes etc.
For training... as much XC running as you can do. I would say the single best exercise you can do to help with the obstacles is pull ups.
Set off running fast at the start otherwise you will be stuck behind queues of people who appear almost stationary trying to go over the obstacles!