As far as single vs. double track, on a smaller layout double is generally better, since there isn't enough room to have enough or long enough passing tracks to make it workable. In this space, you could probably go either way.
I know you say you've bought layout books, but have you bought THE track design book, Track Planning for Realistic Operations? If not, you should get it.
To run the equipment you are talking about in the timeframe you are talking about, I think you need to be thinking a branch line, and 4 axle diesels, making the single track main feel right. Otherwise, I think you'd want more, and larger cars. Actually, I think it works out pretty well.
Given that, I think that aiming for a minimum radius of 14" is probably about right. Using TPFRO (the book from above, too much to type) that gives us 'squares' of 16", so the space is 6x7.5 squares, which is pretty good sized. I think a G shape would make sense in this area, generally the shape nkp drew. I'd put staging along the longer open side, I think, behind a block. That way it is hidden, and yet totally accessible. I'd try to separate the loops as they go through the same area by a few inches vertically, i order to make it look like two different scenes. I'm not sure I'd try for two major towns, unless you want a real point to point (which could work well). By the time you get turned around I don't know that there is really room with out them seeming to be right on top of each other. I'd have one main town, with the yard and engine servicing, perhaps with an interchange to the main line. I f you wanted another town, maybe it would be a small passenger depot and team track.
I'd really like to suggest doing some doodling on your own. There is a layout in your head, you just need to right tools to get it out!
Jeff
, so i am asking for some help.