Re controlling the vehicle from the companion computer, we have some instructions for Copter here. The commands work for Rover as well though.
I wouldn’t worry too much about overheating. I suspect keeping the components dry will be a bigger issue. I use boxco boxes on my boogie board boat.
One thing you might need to be careful about is keeping the odroid away from the GPS because it definitely interferes with the GPS signal. RPI3s produce less interference I think but don’t have as much CPU power.