Marc,
I like Dave's idea of taking the yard out of the middle and going with the L-shaped look.
If you recall, my new "coffee table" will be DC just because the engines I want to run are not available, but believe me, if the Shays came DCC ready, I would go that way. The NCE Power Cab is around $179.
As Dave said, you could sell your DC stuff, engines included, unless you are in love with what you have, and buy a couple of the new DCC and sound equipped diesels from Precision Craft.
As for the sectional track, it will restrict you on curves and also give you lots of chances to misalign the rails. It's great if you like to hear the click-clack of the wheels at track joints, but murder on expansion and contraction as it gives you many more opportunities for heat and cold to affect the track. Gentle undulating curves can only be accomplished with flextrack.
As for the flat layout, why are there bridges shown on the plan? You could raise the entire layout above bench level on say 2" foam and cut out things for the bridges to go over. That way the layout stays flat but the scenery undulates under and with hills added, over the tracks.
Now that we have filled your head with ideas, sleep on it all and then sleep on it some more.
TheDesignFadda