you do not need to buy the game

Add-on is not included.

Add on was a thrid party add on anyway,never an official add on.

Nope, it's an official, Blizzard-sanctioned expansion developed by Sierra. Blizzard likes to disrespect past developers they've licensed to when they aren't pretending the games never happened at all, but it was their practice at that time to license out an expansion they could have handled better in-house. They never should have licensed out their IP in the first place, and there's little excuse in the case of Hellfire as they were quite successful by that point and could have put together a team in-house to handle it, coming up with something fully integrated like Lord of Destruction.

This may not be the best place to ask, but could someone advise me whether I should "return" to Diablo with or without Hellfire?

Background: I am of an age/background that I played Diablo to death back in the day when it was new, it was one of my childhood favorite games, I had trouble getting into it some years ago as "abandonware" on a casual attempt but I feel like giving it another shot now that there's a GoG release.

Not sure if the base game mechanics are better or if it's better with the expansion if I am looking for nostalgia/revisitation. This is probably a controversy.

Point: The game now comes WITH the Hellfire expansion included.

Fair point. I'm guessing I'm looking for some magic answer to try with/without and not worry about making the "wrong choice", when the non-lazy answer would be "try both and decide".

It's like selecting between the pure experience I remember and wondering if I'm missing out on cool new mechanics, or worrying that the new mechanics aren't all that great and it's "not the same" for nostalgia factor.

<shrug> First world problems indeed. :-)

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.

Note that you can take your character (level, stats, spells) and his/her gold into Hellfire, but not any equipment. Sell your gear before you transfer, and use the gold afterward to buy new items in Hellfire. You may want to back up your Diablo character, then sell his items, copy the item-free character into Hellfire, then revert your plain Diablo character to the backup so that he has his items, in case you decide to use him again in Diablo.