MARS mod is technically not legal but it opens up the option of using gmrs channels on your ham. I assume it's because it would allow you to broadcast with at a much higher power than a frs would without the correct programing. A simple $20 frs radio will work also to communicate and I often carry a few to hand out or to use when out of the vechile in a winching or spotting situation.@HornedToad added you to the maybe list!
As for coms i would say CB minimum. The others are just super handy and nice to haves. I am going to try for my Ham license in the next couple weeks so hopefully I will have everything setup before this trip. What's the deal with the MARS mod? It seems to be a win win but there are a lot of Ham guys who frown upon this.
For my radio the Mars mod was completed by removing a tiny diod for others it may be removing a wire. Gigaparts will sell you a radio with the mod as well of you don't feel like doing it yourself.