I'd be remiss in not offering the Lledo Days Gone
line as a good source of vehicles for the layout:
File comment: Lledo Rolls Royce
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There are many nice models in the range, although the majority of them are scaled at 1:64, which would make them best suited for S-gauge, perhaps a bit undersized for O-gauge. Move them to the back of the layout and forced-perspective can keep them looking appropriate.
Seen here is a brace of Rolls Royce models sporting different paint-schemes. Lledo went bankrupt in 1999, but Corgi bought the name and continued manufacturing the range until 2007 or so. There is still a lot of product out there, but its getting progressively more expensive, i.e. collectible, as each year goes by. Many can still be found for less than ten bucks. They made a lot of promotional stuff too, which can be really fun.