I live in the UK and so will discuss the choices I have made which was to start with UK companies/areas and timelines.
I won’t be building a specific era but roughly from 1950 to 1969. And my choice of engines are eclectic. I am building this layout to please myself so will mix traffic from different regions and eras.
I have already bought enough engines for now to run on my layout. I particularly like the A4, so have bought the “Great gathering” from Hornby, but I also like Southern engines and I have a number of older engines that I will try to convert to DCC.
It is possible to split the whole timeframe from 1804 to modern day as different Eras. Explained here by Hornby and here by Bachmann
I have chosen from Early British Railways to Late British railways. I think this is quite a popular timeframe and allows the modeller to run steam and diesel either separately or together.