I would be inclined to give Montagues a ring on the rear belt question, if its like mine, then there is a square section tube running from the floor and terminating inside the roof at a plate with two largish bolts, on that tube is the mount for the O/S lap and diagonal belt. It sounds like all you need is the belt
Deffo on the rust treatment especially the inner w/arches,even if it has been done it wouldn't hurt to get it "topped up" have a search here for methods used., look on it as an investment. You may want to get the cam belt done if unsure as to its condition, I would suggest reading up on the cooling system so you can make any garage aware of the "special needs" of the Bongo bleeding process.
I agree with your little one, Bingo Bongo sounds good.
Hope the weather holds up for you.
Good Luck.