I reuse the screws I pulled out for anything that will be reattached to the body and ribs. Stick with steel, NOT stainless steel. Stainless steel, while providng great corrosion resistance, is a weak steel. I spent over $100 on stainless hardware only to shear 75% of them in half. For bolts, I'd go with what was originally there, usually grade 8 and sometimes grade 5 in certain areas. Grade 8 is super strong but brittle, not like glass but would rather break than flex some. Grade 5 will allow more flex.
I will be using stainless hardware for putting together items within the bus, like cabinets, hinges, etc., where the torque isn't going to be so high.
When it comes to rivets, personally I'd say DON'T. Unless you have an aircraft grade rivet and riveter, then you're just making avenues for water to enter. I will be using the center-post type rivets on select items within the bus, but not for anything attached to it.
Of course, you'll want to coat, inside and out, anything that penetrates the body, including the floor. I use 3M VHB tape, lots of primer, caulk and rubber coat such as plastidip and underbody coating.
I'm no structural engineer or anything, just learning as I go and already want to change things I've done that I really can't undo at this point.