Hellfire doesn't change the base game mechanics so much as it adds to them. The game is short enough that you could do a first play through with the base game, then do a second run with Hellfire.
You can also copy your Diablo save file into the Hellfire folder and rename the file to end in .hsv
to import your original character into Hellfire.
Sweeeeeet. This is the answer I was seeking I think. Thank you for the info.
I know about the Monk but I figured it was maybe like the Diablo 2 expansion where it adds a bunch of new systems to the base game as well (and I think it does, I vaguely remember that it adds a bunch of new spells and probably set items and random modifiers to items and etc), and tbh I'm only interested in those mechanics if they are good and not mediocre.
Now that I know that I can start a base vanilla save and backup the save game before importing the character into Hellfire, this is what I will do: I will start Diablo vanilla, get my nostalgia on, then when I get a bit bored I will backup the save and activate Hellfire + the Belix mod and decide if I want to switch back or not.
if you create char in Hellfire, you can still avoid or ignore Hellfire areas completely and go down to Diablo like before, but you then have the added option of going to the Crypts or Hive that you may complete before Diablo boss, since killing D ends the game. it's a short game nevertheless, compared to modern games.
if you want to play vanilla Diablo, Bnet/multiplayer mode is great because you can go to nightmare/hell difficulty after killing Diablo, while in SP it is more of end of the game.