Welcome to the Forum Obstinate Cymric.
1940's-1950's in Mid wales would be quite an exciting era to model as there were quite a few pre-grouping locos still around. However modelling in N you would have to scratch build most of them!
The last of the Cambrian 0-6-0's were not withdrawn until 1954. Two Cambrian 2-4-0T survived into 1948 and the delightful Liskeard and Looe 2-4-0T survived into 1948 working on the Tanet Valley Line. Then there were the GWR Dean Goods and the Dukedogs as well as more modern post 1923 GWR types including the Manors and the moguls. Some of the LNWR Coal Tanks survived in North and Mid-Wales until well into BR days. Liveries would be mainly GWR/LMS with some BR if you go post 1948.
If you are not commited to N gauge you would be better off with 00 as all the above are available as kits or ready to run apart from the Cambrian 0-6-0.
The recently republished "Tanat Valley Light Railway" by Mike Lloyd (Wild Swan Publications) is quite good to get a feel for pre 1948 GWR in Wales.
Many other books unfortunately concertrate on post 1948 photos although are good for photos of Stations etc:
Steam in Mid Wales by Michael Hale (Welsk Railway Reseach Group,
www.wrrc.org.uk ) has maps and a good selection of photos.
British Railway Pictorial Cambrian Lines by Rex Ghristiansen (Ian Allan) has a selection of early BR photos.
I have some more material but will have to look for it if you are interested.
Sounds like an interesting project.
best wishes,
John H-T.