I wouldn't ride woods with anything less than an actual CP. Some of the pros run just the roost deflector, but not worth the risk IMO. I run a fox airframe and have been pleased with it, stays put pretty good and isn't too bulky.
Forearm, elbow guards: I don't like the kind with straps that criss cross on your forearms, caused me arm pump quick. I have some that have the mesh on the forearm. However, I rarely wear them anymore while racing. Lots of reasons why, none real good just my personal preference.
Knee: I would love to be ridng with full fledged knee braces, instead I ride with some EVS Option (i believe they are), knee guards. I also don't wear these all the time. during the really hot summer races, I just couldn't bear them, I would end up being more uncomfortable and focused on them instead of being focused on the trail. Staying strong and focused will go along way in staying uninjured, although I know nothing is a given.
I have also tried the full armor jacket thingy.. I can see where they are nice, but it wasn't for me.