If you’re looking at one of the old green planers I suggest you avoid the direct drive version. You can identify these by the fact that the motor is sticking out the side near the top of the unit. Not only will a belt driven planer give you a smoother finish due to the isolation of the motor vibrations, they are also more compact.
I used to own an 18” Powermatic, which was actually larger and about three times heaver than the current 20” version.
Man was that thing HUGE! But I digress.
Bonus features, but not deal makers or breakers would be things like a dust chute, a mobile base or a built in blade sharpener. Killer options are things like a spiral cutter head. These cutter heads are quite and uber-smooth. With a spiral head it’s critical that either the blades are individual carbide cutters, or there MUST be a built-in sharpener.