Paint questions

[ShinyC10]

First attempt to paint my truck (86 swb fleetside) on my own in the future. Currently have the cab off and on a dolly while I do all the body work to it. The plan was to get it in some TAMCO 53xx primer, paint jambs, inside of doors, inner fenders and then put back on the cab to paint the exterior together.
Few questions:
1) The cab will be sanded to 180 then sprayed with TAMCOs 53xx primer. The bed, tailgate, doors and fenders have an older gloss black paint on them. What do these need to be sanded with to apply paint or should I spray a sealer then paint?
2) Is 400grit a good finish for paint? Haven’t landed on a color yet but I’m a new by so leaning towards easy colors.
3) Seems like the easiest paint steps are to paint all the jambs, and inners. Reassemble truck. Then paint the exterior. Is there any prep to those inner areas before spraying the exterior? Any adhesion issues with that?
4) Is cutting/buffing a mandatory step when you finish painting? For some reason, Im just as nervous for that stage than painting to truck.
5) Lastly, how much material to order? I’ve heard everything from 1 gallon of base/clear to 2 gallons of each for the new guys.

I’m sure there will be more questions but these are the top of my list at the moment. Thanks in advance.

[65truckowner]

One the older gloss black I would sand,prime, then block sand then paint. I use 400 grit before painted but I use single stage paint.