Jaime
I think you have the right idea - get your design right and then put it together.
Totaling load and doing the calcs on paper prior to purchase is the most economical in terms of time and component cost.
Put the unknown loads across 13 or 14 volt and read current. Now you know max current.
Add 10% to the total to ensure you have covered what you may have forgotten.
You may be a bit high when everything comes togeher but underloaded alternators and motors last longer.
