(Click images to Enlarge)
Structures probably could be made from O or S Scale kits. Since no one I know can afford them, there are other sources. There are many HO scale structures that can be used. Structures won't have to be scratchbuilt. Print out our scale rule and measure structures at the hobby shop. There are more Scale55 structures available than one can imagine.
Whitewash City offers paper buildings in Scale 55, ignore their 1/60 scale. Print the saloon out and make one of cardstock. See what Scale55 offers in size.
Buildings Unlimited(Ameri-towne) offers inexpensive plastic O scale buildings suitable for Scale 55.
The doors will need a typical transom added to reduce their height.
This garage will make a nice freight house.
O Scale Structures for 55n3