Congrats on putting together your first rig. I still get excited working on mine. I remember asking a lot of the same questions at one point. I will share with you what works for me.
Best pump? Air pump. No experience with 12v.
Gun? I actually use a stainless steel pressure washing gun. Just in case I drop it, that way it won't be spraying chems all over. I have about a year of use on it so far.
Mix consumption = At first you will probably use more than you need until you get the hang of it. I wouldn't schedule two roof jobs on the same day with only a 65 gallon tank at first.