For beginner students that are 14 or younger I recommend smaller sizes if possible. Small tom sizes are crucial because the student needs the toms closer to the bass drum for height reasons and ease of practice and performance.
20″ kick, 14″ snare, 10″,12″ tom toms and 14″ or 16″ floor tom. 2″ difference between each tom tom usually has gotten better sound results.
The minimum number of drums would be 1 bass drum, 1 tom tom, 1 floor tom, 1 snare drum, 1 ride cymbal, 1 crash cymbal, a stool, a music stand and a metronome.
When buying used, make sure the drum heads are in good shape and not dented torn or pitted and that the cymbals are not cracked. Make sure the stool can be set at a comfortable height for you (especially for younger students who needs the stool lower then most stools would allow).
Make sure cymbal stands are stable. You don’t want the cymbal stand falling over and cracking, so when testing out the kit make sure to hit the cymbals like you would during practice and watch how the cymbal stand reacts.