I never made it very far with my build yet, having two kids and having to move took a lot of time away from it.
I started with power, bought the battery tray for the diesel trucks, battery, isolator, distribution wiring, fuse blocks, etc.
Added communicaiton, lights, etc.
Added the cap and fixed it up with lights, sleeping, etc
gathered up a bunch of tools and storage in the process
Better tires, skid plates, etc.