I've googled 'Hellfire' and read as much as I could about it, but I'm still unsure about its stability. Everywhere I've looked, it is said that Hellfire is kinda bugged and that a lot of fixes from the original vanilla game were never implemented.

So, how buggy is it?

I've finished my first run of the original and I loved every second of it. It's been so many years since I've played it in my childhood. A trip down memory lane, that's for sure.

tl;dr

Owning both a hard copy of Diablo/Hellfire and the GoG version I'd say...

Original discs on Win 10: some issues with cutscenes and weird graphical artifacts, mostly on pause menu but somehow there's better graphical fidelity.

GoG version: No issues with stability of any kind have I discovered in my playthrough (so far) but the graphics such as sharpness and brightness are somehow worse.

Longer answer:

I originally got Diablo about two weeks after it's release (90s release.) I still own the discs of Diablo and Hellfire and have had them installed again for about a year or so (Win 10 machine.) I picked up the GoG version during the summer sale, copied my save file over to the GoG version, booted up Hellfire and it looks... bad. Even after fooling with the new graphics options (you can easily get graphics mods for the disk/non GoG version as well) it still looks bad. I exited it and booted up my save file on my original disks and it looked so much brighter and cleaner... so I could actually see everything on screen and it was much less grainy.

If you want to buy the GoG version I'd say do it... it is a little more stable than running my discs (On Win 10, at any rate.) Running from the discs I have occasional weird graphical artifacts and some tearing/weirdness during the cutscenes. I have not noticed this with the GoG version. To be fair, I've only played about 45 mins of the GoG version so far, but I've had no issues. The running speed in the GoG version of Hellfire seems a little too fast compared to my disc version, but I could just be crazy.

Heck, for $10 you really can't go wrong, even if you never even touch the expansion.

first, Hellfire is based on Diablo v1.04

you can read Diabo changelog and basically everything from 1.05 to 1.09 is what didnt make it into Hellfire, i worked with Hellfire more than its creators, I can confirm it's absolutely true

bugged doesnt mean it crahes all the time, Blizzards and Sierra crews didnt see eye to ete about the expansion, Sierra were not given enough time to work much on it, and the end product couldnt be a quality one given such time constraints.

we worked for years and still keep working on it, the mods The Hell 1 and The Hell 2 for Hellfire are what fix all of Hellfire and Diablo bugs, even those that were never fixed in the latest builds of Diablo, you wont get nostalgia from that, but the fixes are there in such amounts that no other mod ever comes close to that level