Internal Parts you CAN powder coat:
tub
shelves
I call them bottle shoes, (but the part of the bottle stack where the bottle rests before vending)
the back Spine of the bottle stack, rests along the back wall of the tub
and Face plate

The rest of it you should consider cleaning, polishing , or zinc plating , or for budgetary reasons , some parts can be painted..

in all honesty the small internal parts really dont' need to be redone unless they are all rusty .. but you should be alble to clean them up pretty well and reuse without replating..
talking about the H shaped slides in the backside of the bottle stack, the S Shaped levers,, and small springs, and parts that actually make it work.

now as far as the vending arms, I've seen some guys plate them, however you have pot metal ends that does not do well with plating, others clean and paint, some dissesmebly and zind the arms but just clean the rollers.. so it's really up to you on how extensive you want to go..

Good Luck! Hope this helps

Nice summary John. One  thing about zinc plating, if you do want to plate some of your internal pieces, most plating shops usually do plating by the batch not by the piece, so the price will typically be the same no matter how many pieces you take in.  Also, lay all your parts out and make a photo log of the pieces you take in so you can make sure they all end up coming back. I had all the moving pieces of 2 complete bottle stacks replated in one batch for about $100.

Thas is all great stuff...........thanks so much for giving me more to hink about, Zinc plating, poweder coating, anondized..............to much to compute.........