Keeping in mind you probably want to stick to champs that are able to engage from range since that is the distance you are accustomed to attacking from I will throw a number of different options at you.
Tanks: These guys are big and beefy. They aren't going to die so easily but they aren't going to put out the damage you expect them to.
Good engage and AA's are essential to his passive. He will help you adjust w/o having to worry about getting in close.
His passive will sustain you through the poke you aren't used to avoiding. He also has a big movespeed boost and is both safe and aggressive. A good champion in general for learning to play Melee.
Bruisers: These guys are gonna be closer to the damage you are used to dealing but they tend to be a bit more complex than Tanks.
He has both damage and a gapcloser. Definitely a good choice for learning the role.
The hardest part of melee champs is dealing with kiting. Olaf will just burst through that so it is one less thing you have to worry about.
*I simplified the classifications because I wanted to prevent confusion and organized by the role they feel most like.
The champs you mentioned.
Poppy is a fine choice.
Mundo is good if you just want to dip your toe since you can fall back on your ranged methods with his Q, but he isn't strong and if you want to really learn melee champs he isn't the best choice. Up to you.
Panth is good. He gives you his Q to fall back on and his passive will help prevent harass.
I would steer away from Diana since she is an assassin, which I am guessing you aren't used to playing and they come with their own range of difficulties, so I would stay away from them. She also has one of the most unique and unusual skillshots in the game and hitting it is core to her kit.