(This will probably be revised later. I’ve just written down what was on my mind at the time)
Modelling a preserved railway is probably best for a first layout because it can be whatever you want it to be and it’s a good way to learn modelling skills.
I’m modelling a fictional heritage railway set in the fictional area of Castle Rock for a few reasons:
One is that it is easier to acquire O gauge models without an era or region restriction so I can get whatever models are available.
Two is that I like to write stories. A fictional place is the best thing to write about because you can put A N Y T H I N G you want into it because it isn't real in the first place.
And Three is that heritage railways have rolling stock that the past doesn’t have; like wagons carrying bits and pieces or miserable looking skeletal coaches in cramped sidings or tanker wagons that look like they’ll deflate or the sadness of a lump of iron that used to be a wonderful steam-beastie of the railways just being left in the corner siding to rust away like it was never there. These examples provide something interesting to model, especially if you ever get your hands on already broken models.
Or you could model something a bit nicer like a heritage railway with locomotives and rolling stock that is very well looked after. There’s many examples of that.
Modelling preserved railways doesn’t need to be accurate either, since it would be different from how the railway would have been in the past.
Thats just my take on the subject. Opinions will vary.