I've played plenty Diablo clones, but they're all different, and this isn't just a mere Diablo clone anyway.
I was planning on starting on the hardest difficulty. For someone who's getting into this evidently huge game for the first time, which character is most fitting?
As far as I can tell, since they have access to all the same skills, they only differ in starting stats and how much each stat point has it's effect.

just play the game... experimenting with characters is part of every rpg.

No set characters, really. However, you can insta-spam spells (like Fireball) while adding either frost (slows enemies down) or poison to your melee/ranged attacks.
I can't say how you should build your character, just know that you have tons of options on crafting a character that is perfectly suited to your playstyle.