Organized in rough order of getting you safely down the trail (and back again) in my opinion.
1. Tires
2. Emergency & Communication Gear
3. Bumpers
4. Skidplates & Rock Sliders
For more specific answers check the individual forums of the board (i.e. tire selection, etc.)
You can also try perusing my and other owner's websites to see what we've done and how it's helped us.