I think I saw you ask GRJ about tmcc. It is basically a board that can control a can motor (two wires), light(s), and a speaker. Aside from those wirings, input from rollers and chassis(or wheels) too. Finnally, and antenna (wire, a handrail, etc ) Boards? Sunset 3rd rail. (or used engines/boards) The latest on where else, etc, I'm not up on that. Lionel ERR co closed last year. Scott at Sunset is the source now I think.
All of the above mounts into the tender. A tether runs from engine to tender containing motor/light wires etc. and roller power wire to the tender (sound might be omitted for more space)..Thats four wires bundled. 2 for motor, one for lamp, one for roller power (other "#2 wire" for light is chassis or roller, and tender tender has chassis ground too, so just 1 wire also) Adding another roller pickup to the tender is a good idea. Short engines can stall on switches, etc, and an extra roller helps tons in seeing the engine always has power.
Wood, metal, or plastic. Maybe all. Hardwood makes a decent frame if you don't mind running ground wires. I've even used balsa or bass. Metal frames can ease wiring and allows easy use of nuts/bolts/rivets too; not just screws.
Plastic takes way more reinforcement effort in a chassis. But makes a great body, easy to paint and finish nicely. Wood has grain to bury and porosity, so takes more finishing effort. Tin/steel finishes nice, but will be harder to form or glue. Aluminum, pita here, skip it.
Brass and solder is the craftsmen level.
Actually, plate metal, even aluminum, is a other chassis option. Quick and and easy, hefty for added weight, machineable for bolts, etc.
Before I forget, you hadn't considered an engine mount oil tank for fuel
I bet the tank on the boiler is partitioned, or the fuel tank is slightly hidden Loggers could almost use thier loads as fuel, and it doesn't take a lot of coal to fire a tiny boiler, a small bin might be ok. If the stops aren't far off, coal might be added at the stops only...? I don't know for sure, just guessing.
So, I'd go with a plastic body most likely. Low cost, easy to work with, strong enough, rivet strips are the same material too.
The frame? Whats handy?
"Whats handy" determines alot around my layout