The tunic on Bill Shea's site would be a very good buy. I'm not sure what you mean about the collar not being flat. You have to remember these tunics are over 65 years old. They tend to shrink a little so they might not sit like they did when new. You can always call and ask Bill if he could bring the tunic to the Meadowlands show in Union in July. He's almost always there.
The visor that's on Craigs site is for an officer and wouldn't go with this tunic.
NCO's (Non Commissioned Officer) are enlisted men. They are just of a higher rank. Corporal is the lowest NCO rank.