Machine
A thickness planer takes a board with one flat face and makes the other side parallel to it, at whatever thickness you set. Rockwell's 13 x 6 in 22-401 is the classic here, with a three-knife head and a variable feed driven by a corrugated infeed roll and a smooth outfeed roll. Ask for a video of it planing a board. It should feed evenly without stalling, and the knives should leave a clean surface. Check the motor plate for phase, and ask whether a spare set of knives comes with it.
1 model we cover.