Hi Ash and welcome.
That's a lot of questions there and much will depend on what you would like to do with the layout.
At the lower end of the mountain are various railway sets, with many themes to choose from. These can be fixed on a board very simply. A look at Hornby {Arnold}, Bachmann {Graham Farrish}, Peco {N}, will give you an idea of what sets you might like to see.
There are also lots of scenery items which can be bought, ready to place around the tracks, as might please you.
That said, I rather get the impression that a ready-run tracks laid board is what you are looking for, ready to use. There are several companies which can make layouts for people and can be found with a google search, for layout builders. {It is advisable to look vary carefully at what they produce and comments of those folk who have bought them.}
Have you got an idea of the type of trains you would like to see running, as that would give people an idea about what they might recommend. Oh, yes, and what sort of size area do you have available? {Conservatory's vary quite a bit in size.}
Julian