1/2" or better thickness (depends on how far apart the bracing is, but I'd start at 1/2")
Positioning from the wall you must look at the curve dia overhang on your worst offending rolling equipment, as that overhang continues into the straghts a few inches.
Track brand, I'm partial to Super-O and GarGrave, but tubular is fine up on a shelf. I also really like the long track pieces of the latter two.
Tressel spacing goes 1 per joint. The joints need support.
For one 36"-40" I'd use 3 or 4 as the length can twist some as well if not supported. (some locos and stock are very heavy, some light; also a factor.)
Play with the 4% grade on the floor with short and long trains. It still may be steeper than you think. Pay attention to how much of your attention to the throttle is needed and if that suits your real life run style.
(I wish I could've managed less, even 3.5%. Those PW over unders are small trains that use the curve geometry to a traction advantage as well. It's a careful balance on those. (I guess maybe I just like long trains; eight to ten cars to climb is a little shy of my norm on flatland tracks.)
