I've been meaning to add my own thoughts about building a layout. It's very much at the "armchair modelling" stage of the project and not at all high up on the priorities list, but here goes.
Era - blue/grey - probably more early 80s than 70s (I reserve the right to add a bit of colour if I so desire). Scottish region, of course, and set "somewhere in the Highlands". I haven't quite decided which line yet, although I had the thought that basing it on West Highland or Kyle/Far North practice would eliminate the need to have "proper" signals. However, I may decide to model a different line in order to have a bit more flexibility in what I can run.
It's likely to be single line with passing loop, probably with some sidings to cater for some nearby industrial site or engineering trains and a disused goods yard with a distinct whiff of decline about it.
I have a thing for Sulzer type 2s and I would like to see class 25/6/7 operating with Mk1/2 stock, perhaps with the occasional 37. I realise that the earlier types don't really mesh well with the era of Radio Electronic Token Block (RETB) signalling, but nothing is set in stone yet and I'm willing to apply a bit of modeller's licence anyway. Short trains will definitely be the order of the day as I don't have a lot of space.
My main priority is de-cluttering and freeing up that space, which means construction will probably commence in the early 2030s...