Great project! Best of luck with this.
The danger on here is that you will be faced with more choices than you probably want, but here goes:
1. As someone else has said, you may want to consider using the new Peco Code 75 Bullhead track, which will give you perhaps a far more authentic look than you will get from Code 100 flat bottom, set track. The downside is that it is more expensive, and only the large radius points are currently available. So it depends on how important the look of the track is to you.
2. Have you decided what era you are modelling? if current, then a Class 33 diesel is probably a must, as this does many of the stock moves and occasional, out of season passenger trips. If earlier than around 2000, then forget it.
3. Strictly speaking, under Southern Railway / Region rules, you should have a catch point at the exit to your sidings at the bottom of your plan, or have a further siding off then instead, so that the point can be set to divert any runaway from the sidings away from the running line. But you may not have space to insert that, in which case, apply Rule 1 (It's your railway, so you can do what you like).
4. Coaching stock - you have not mentioned this (unless in your list which I have not opened) but there are a few useful new items about to emerge from Hornby particularly, so ensure you know what you want.
5. Have you decided upon live frog points (switches), so your Terriers in particular do not stall on them?