I haven't played in a while but am going to dig out my copy. I'm fairly certain it doesn't include Dark City. There's no option to purchase the expansion by itself. Does that mean I'll need to buy a new copy of the whole game to get it?

Unless you can find it on BGG or ebay, yes

From the comments and reviews I've read you're probably better off getting the new set, as it seems the quality of the cards and pieces are much better than in the old.

I agree. I own both and gave the original to a poor family that runs an orphanage. I have done the same with Twilight Imperium second edition and Runebound 1st edition.

I also gave them all my milton bradly games once I discovered FFG games.