1. Yes, it's perfectly fine to update. I was not aware ICS was available just yet though.
2. The battery will do that for the first few days while it adjusts. After that, it will last much MUCH longer (mine lasts almost 27 hours even after extensive web browsing).
3. You really don't need any. All you have to do is install apps you know are safe and check the permissions they want (i.e if a game wants access to your contacts, there's something odd about it).
Hope that helps.