It certainly is a challenge and it would take a long time to find enough All Nation kits to make an decent sized consist.
For that reason I am willing to sacrifice prototypical accuracy and will be using 5 MTH cars to do the job. Most people who visit my layout are not train buffs (friends, family & neighbors who visit around the holidays) so they will not know what is and isn't accurate. As the old saying goes: "What they don't know won't hurt them!"
Not to get too off-topic, I have been on the hunt for a MTH premier CNJ pacific so I can run The Queen of the Valley. Once I get that loco, I will definitely need to get some prototypical cars.
Bryce
The All Nation kits are not that hard to find. I have seven coaches, two baggage \ coaches and a baggage car. I tracked all of them down in just a few months.
A correct BC diner is an 80' prototype with a truss rod support system. I have the plans for it and will likely find a reasonable Walthers donor as a start.
The real work is in building them well. As for an MTH premier Pacific I have one 3 rail version that was going to be a donor for my 2 rail partial one but I liked it the way it was and I am seeking another. It still needs a lot of work to be correct.
As an aside I also have a Weaver Reading GS1 Pacific that is closer to a CNJ 820 series Pacific. That Pacific type has the honor of pulling the duty when the BC derailed. It has the correct Wooten firebox but needs some details updated.