Armor. Transfer case skid/crossmember, bumpers, sliders. Unless you dig a ton of sheetmetal damage.
A locker in the rear to start, gears, and tires.
Make a plan for what you want the final product to be. You'll save money buying the right part the first time, rather than changing things as you go. For instance, don't buy a single case crossmember if you know you will be doing duals down the road.
Good luck.
