If all that load calculation is too much or you're unsure, you can just start with 100 watts and see how that goes, you can always add more solar. You might want to get a charge controller that can handle 20 amps so you have some room to upgrade.
More than 100 watts can get to be a problem for hauling around and storing so maybe get a 100 watt portable kit with a 20 amp controller now and if you want to add more solar you can put that on the roof. If you mount the controller in say the front storage compartment you can connect it to the battery (via a fuse) and run all your solar modules over to the controller. It doesn't hurt to add solar modules of various sizes and/or tilts together as long as you combine them in parallel electrically.