Just a couple of quick observations. The Walmart engine is a 4-4-0 American which is a bit more synonymous with the early west. The Goldlok is really an 0-6-0 which is usually used for switching operations in a yard and would appear quite a bit later in the historical timeline. That being said its really all about getting some moving terrain on the table so ultimately it doesn't matter!
You don't really need to do any work on the passenger car. The Walmart version is simply a closed vestibule version, although its probably more common in the east than in the west. So if you want to take on the project feel free, but its not entirely necessary. Of course for the American West all the freight and passenger cars should have two trucks with two axles (4 wheels) per truck so all of the cars are a little on the short side. But again sometimes its all about getting trains on the table. Personally I would go with the Walmart version since it uses the 4-4-0 American, but its nice to have some variety too.
Nice Find!