As suggested by Roatan Man, it is the overloading of new/difficult situations/stress factors that makes Cocos challenging. When overly stressed, a diver might tend to use up air faster, and when you consider how expensive these trips are getting to be, why not wait until you gain more experience so that you can get the most out of every breath of air. In extreme cases, an overstressed diver might panic and that could spell trouble not just for him but for those around him.
Of course you could be lucky and spend a week there, find the so-called challenges to be overhyped, and think nothing of it, as was the case with BDSC. Don't be misled by that. The thing about Cocos is that conditions can change dramatically from day to day and even from dive to dive.
I was also there in August this year. Conditions at most dive sites were about average, but there were strong currents and washing machine conditions in Manuelita Channel for 2 days that contributed to an incident on one of the other liveaboards that required evacuation. Fortunately for most of the remaining divers on that boat, another liveaboard was heading back to the mainland that afternoon and took the victim along with two Good Samaritan divers who, as doctors, chose to care for the victim while foregoing their own dive trip; otherwise, that boat would have had to lift anchor and pangas and head back, thereby ruining the trip for all.
And on a more personal level, consider also that if you were to get into trouble, a chamber and good medical attention are a good 40 hours away.
As far as best time to go, I prefer to go July-September. It is said that late March/April are good times to see baitballs. Unfortunately, the Cocos National Park has banned diving with baitballs.
And as far gaining more confidence, in my opinion, getting more dives under your belt is probably the best approach. Do Cozumel and Tobago, as suggested by Roatan Man, to get more experience diving in currents. Eventually consider doing one of the liveaboards that go out to the Revillagigedos or Socorros Islands of Mexico. It will give you a preview of the long ride out and back (24 hrs) in conditions that can be rough, and will be a good stepping stone for doing Cocos. Not to mention that the diving in the Revillagigedos Islands is pretty awesome in of itself. You are apt to experience many of the situations Roatan Man mentioned: currents, surge, rough seas, and back roll negative entries and. But not as bad.
