To sum up what has been said...

The surface of a hollow core door may be a little bumpy, but in the grand scheme of things, it is flat.

The surface of a piece of Drywall is smooth, but if you lay the board on something that's not flat, it will conform to the shape of whatever it's laying on.

So to me, a Hollow core door should come first. Then if you want to add a piece of drywall, or a ceiling tile on top of it, go right ahead.