While I am sure a drum sander would be a nice tool to have, is it necessary for the
above goal?

No. Just plane it. I wouldn't use most drum sanders for dimentioning
anything. It would take a sander with some serious muscle to do that.
I want a drum sander also, but it would be fore getting a certain
finish while reducing sanding time, not dimentioning. On the other
hand, if you get some seriously ragged wood, like maybe some old barn
wood or something, it might be better to run it though a drum sander
first rather than a planer, since grit and old paint in the wood
wouldn't do much to a drum sander, but would dull or nick planer
blades.

brian