I understand GOG's version of this game is partially broken with respect to the pawn rental system and some quests. I don't care for the multiplayer portion and I care more about the DRM-free version.

1. Can I still 100% the GOG version of this game in terms of both beating the game and getting all 59 achievements?

2. There are over 400 mods for this game. I'm looking for a definitive HD upgrade experience of the original, including QOL upgrades. Any recommendations?

Thank you!

I will be buying this and probably sticking to vanilla. Thank you again, sir/madame!

Yes, it's definitely possible to finish this version of the game totally single player (I did this) and 100%'it. But, as Langer said, it will take longer to farm Rift Cristals (In game special currency for Pawn system and items.).

While in Normal Mode, you get Rift Cristals for killing campaing monsters, in Hard mode you don't, requiring you to spend more time in Bitterblack Isle (BBI) which is End game content (Really good and fun end game content imho.).

The best experience for the game is in Hard mode for me though, as you have to constantly manage your pawns and stamina, requiring more thinking and item management before each encounter, at least in the start to mid game.

Talking about mods, I personally recommend World Difficulty 1.5 and 2.0 for New Game + (After you enter their Discord) by Lefein.

There also good quality high fidelity graphical mods in Nexus. But this you can choose by taste, as some just add shaders, while others improve the natural feel of vanilla textures. (Which are awesome by themselves imo.)

This is really awesome to hear. Thank you so much!

Just to clarify as my post may seem misleading about Achievements:
I haven't played to get them. By 100% the game I mean doing all single player content without problems.

In this version I haven't tested myself:

Online Ur-Dragon (End game boss that can be played with other players) - but there are still one weaker Offline Ur-Dragon version.
Online pawn hiring system (But the game generate offline Pawns, which scale with your own level as you progress, that can be hired without the need of internet connection at all.)

So if there are any achievements for Online pawn hiring, or Online Ur-Dragon, I believe you will have a hard time completing these.

I'm close to completing 100% of the achievements and I've killed online ur-dragon a few times myself. So yes the online gameplay "works". You can hire other peoples pawns and use them. you can fight the online ur-dragon.

"only" the social features are missing.

no pawn return means:
-yours and other players pawns won't receive ratings and thus there is no competition for "best rated pawn"
-pawns won't keep the knowledge they gained while being hired by other players, so no helping newbies by training their pawns
-gifts and gear you give to a hired pawn won't make it back to the owner, so no trading among players
-i already mentioned you won't receive rift crystals for your pawn returning which is probably the only thing that affects players who don't really care for the social aspects

also I think the scoreboard for online ur-dragon is glitched because some of my kills didn't seem to count.

So yes the game is "playable" 100% but people who are looking for the full community experience should stick to the steam release.

Thanks for explaining. :)