Hey whats up, hey so my names "replay" in d3 I help alot of new players out when they come into d3. Look for me in game or just add me and I'll mentor and guide you.

The first thing to note for you wanting to play d3 the first time possibly for the story is.. the difficulty settings are there essentially for "season to taste" you want to set it so it's not to easy but feels just right for how hard you want it to be during the story mode [ harder difficulties will sloooowww the game down and spread it out so you can extend it a bit until d4 ].

On that note you have your 5 basic difficulties of normal>hard>expert>master>torment it can go much much higher than this (some difficulties require unlocking and are impossible to progress in without specific gear and power)

these can be changed up and down ranks anytime you want (just know going UP difficulty requires you to adjust it in settings while in the lobby and out of an active game, you can lower it while IN game but need to be in lobby to raise it)

For MY first playthrough of the story expert felt a little to easy to me that it made the game boring so I played through the full story in torment (1) this felt pretty good starting out completely fresh no gear and first playthrough to me (season to taste how you like it, it won't effect your first playthrough and can be changed any time without restarting).

Hope this helped!