Even on sale, I'm a little on the fence with regards to getting the game, due to a lot of the quality of life issues which didn't get changed until Diablo 2. Something which might tip me over is an answer to a question:

I'm unclear on whether the new class added in the expansion is playable in the original campaign as well.

Yes, if you use DevilutionX.

To be clear the Bard and Barbarian can be enabled in general. The Monk is only available in Hellfire. But since the engine is unified all fixes from Diablo now also applies to the Hellfire expansion which mitigates many of the issues with the expansion (originally based on Diablo 1.04).

If you just run the game as is from GOG then you can only play the 3 new classes in Hellfire.

Hope that's helpful.

P.s. DevilutionX also adds many QoL features (see the settings menu) that you might appreciate.

all of the new levels requires that you reach around level 15, before you can access them. you therefore have access to all of the original levels either way. also: when you complete the new levels, you still need to complete the old levels in order to complete the game.

That's a good point. The Hellfire expansion also includes all the original levels, though things will work with the slightly modified game mechanics.

Yes, that was helpful, thank you.