Some people stay to an exact prototype and are very critical which gets expensive. Some hit an era and try to stay close. Remember it is your creation and your world you are building and you make the rules. Remember to have fun with it and do what you want.
I model a railroad somewhere between the 40s and 70s depending on which way you are looking. I only run steam and I love old rolling stock. I am trying to get all older vehicles and buildings and whatever, however things are expensive and I use what I have. If I see something I like and can afford then I replace the things that dont quite fit and ebay then for my next thing I see that I want.
I have one guy that shows up once a year. Once he said you have mostly Pennsylvania, NS, NYC and Western Maryland rolling stock. It looks funny to have sante fe and western railroads running in the 50s. I said I like them so I run them. The next time he came back and said I hadnt made it any better. My answer that time was that it was my world and all the rolling stock was leased from all over the country.
Good Luck and Remember to have Fun