Hi, John.
The area of your layout is roughly what I have available. and your design isn't too far removed from my plan either. I'm currently working towards large oval with an inner passing loop, to run a Stainz loco & a few wagons, and at an upper level at the back an independant line for a couple of Bachmann trams. There's quite a bit of layout work to do regarding point control on the main loop, and I've not yet started the higher level. My wife is currently working on some simple stations & buildings and we currently have no idea on infill of plants etc. Unfortunately, I don't have a shed to spare to give me any permanent cover, so there's much transporting of rolling stock (and probably buildings) when the weather isn't too good.
If you'd like a get-together (either in Cheltenham or for the G-Scale show on Feb 9th), let me know.
Best regards,