You can flatten an untrue face in a planer. It takes a jig, but it is not complicated. And once built the jig will, of course, last forever. Once one face is flat, the opposite face is, of course, a cinch to make flat and parallel. Try flattening a 12" wide board with cup, twist, and warp on a 6" jointer - I've heard it can be done but it ain't easy, not by a long shot. In planer, it is a cinch. A planer is idea for stock blank truing.
The 90 degree jointed edge can be done on table or band saws which also do so many more functions.