Chessie,
You are wise to think aboout minimum radius first. It only takes one long-wheelbase locomotive to make one wish for broader curves. Since you have a duck-under and access hatches anyway, check and recheck to see if you can make the "running" portion of your layout follow the outer walls--eliminating return loops entirely. If not, see if one of those return loops can go around the stairway (through a tunnel if necessary). It only takes one pair of return loops to give you an "infinite" main line: putting your main line into ovals in the middle of the room not only eats up a lot of space, it also makes most of your switching inaccessible. The modeler above who recommended taking yet another look at Ross switches is spot on. They are expensive (as all switches are) but they are beautiful and provide the best operation for switching operations. You have staging outside your ovals: it will not be long enough for decent O scale trains, so why not move the loops to the end and give yourself a true yard in the middle, allowing for through trains while otherwise switching. A yard doesn't have to be a giant classification yard: a rural interchange with a freight house, a team track, and maybe an ice house or stock facility can still offer hours of interesting switching--especially if you allow for the through trains to set out and pick up cars.
Sorry to be so "revisionist," but one of the best layout designs I ever saw was an around-the-basement-walls branch line by Jim Six. It was so simple and elegant, yet full of work to do while allowing for continuous running. And it was really adaptable to any scale from N to G. And the relatively narrow benchwork left the entire middle of the room for trainwatching.
Of course, I'm dreaming here about what I would do. But most model railroads I see are overbuilt--especially three-rail ones. They all end up looking the same because they're all a jumble of the same common stuff. Get "Peake" a nice brass Pennsy 2-8-0 for Father's Day and see how that affects your planning . . .
