I saw this post kinda late but I'll answer anyway. I've been a model railroader most of my life and when I started my current layout about 8 yrs ago I decided on code 100, thinking it would make maintenance and reliability a little better. I don't feel that way now. If I were to do it again I believe I'd go with code 83 and handlay turnouts using the fastracks system. I'd also solder up the turnouts on PC board so as to make them semi portable.I can't seem to make up my mind on track arrangement. Besides the prototype changed the track layouts as needed.
I'd go with code 83 and 70 simply for realism sake. I don't care for Atlas turnouts anymore as they've over done the point notching and some of my steamers pick the notches in either direction.
The size of the rail has nothing to do with compatablity with DCC.Think about all the different scales using DCC.
Good choice with DCC a little spendy to start,but operation is great with it.
Deisels will make your life easier as they aren't so picky about the track they run on.
Transition railjoiners are available to change from one code to another,so no problems there.
Good luck on your layout BILL