Best way to drive a 20 kg object on a turntable?
If you are not trying to be accurate (as you allude to) but just continuously rotate something you could probably just use a normal DC motor with rubber drive wheel (have you considered using the belt/motor from an old vinyl record player turntable?).  If you need slow rotation you may need a gearbox (model shop, old toy, meccanno, lego?), or need to PWM (Pulse Width Modulation) drive the motor (remote control car motor controller?).  Another idea is an old variable speed electric drill/powertool, you could rig the trigger to a specific speed fairly easily (cable tie?).  This will all depend on how quick-and-dirty vs robust/reliable/long-term you are aiming for and whether you need variable speed and/or direction control (things you don't specify).  Stepper will give you the most control, but is probably the most complicated.  Match the complexity of your solution to the complexity of your problem. :)