The first thing to remember is YOU make the rules, and pay the bills, so how closely you follow the prototype is your call. Do as you like/can afford.
Couplers....most use Kadees. The trip pin is supposed to represent an air hose, but it doesn't do too good a job of it. Protocraft makes a VERY realistic type E coupler that can be operated with a magnetic wand or a cut lever if the car is so equipped. There are others....San Juan, Monarch... your call as to which.
I've seen several round roof box cars. Pretty sure Weaver made them and different brass importers had them made. Sooner or later almost will appear on Ebay. This hobby requires a certain amount of patience. Immediate gratification seldom occurs.
Go to some 2 rail shows if you can. Try and find back issues of the late "O Scale News 48/ft" and early issues of "O Scale Trains" magazines. The advertising within is a tutorial on what is "out there". Use the Internet and search engines.
Good luck!
Simon
PS: One last thing. Invest in some books with good photos of prototype equipment so you know what the real thing looks/looked like. There are a lot of bad models out there!
