I've decided that the setting I want to run with D&D 5e is Forgotten Realms... 1e, of course. Nothng against the 5e Sword Coast book, which I have yet to check out; but I've spent many happy hours in the Realms, not at the tabletop but with D&D licensed PC games, and the sparsely populated and cruft-free 1e version looks better suited to my DMing style.
Other than the gray box, what's good for pre-ToT Faerûn-crawling?
Still undecided on the exact location but it's a toss-up between Sword Coast (Luskan!) and Cormyr.
If you liked the computer games, you should really check out Ruins of Adventure.
I was actually going to recommend it anyway, but basically it's an adaption of the first game, Pools of Radiance. It's sort of a sandbox where the party members explore parts of the ruined city. There's a vague plot, but nothing overwhelming.
Haunted Halls of Eveningstar is supposed to be like Ed Greenwoods In Search of Adventure (ie, an introductory dungeon). I don't think it's great, but it's worth a look.
(Note: I am not a FR fan, I just have a smattering of old FR products and those are the two that stand out, especially Ruins of Adventure)
A couple I like are Under Ilefarn & Waterdeep and the North. Bloodstone Lands is ok for high level play. My general feeling though is that the Grey Box is already very detailed and extrapolating from it to create your own campaign world is better than relying on tons of supplements.
If you are starting in Luskan, then FR5 The Savage Frontier (http://forgottenrealms.wikia.com/wiki/The_Savage_Frontier_(sourcebook)) is the guide for 1E. I'm using it to start a new campaign for my wife and kids (using OpenQuest 2) and you can easily set up a sandbox using that supplement. It has an overview of the factions in operation along with some NPCs, locations, and news to get you going.
Seconding a few:
Don't let the name fool you, FR1 Waterdeep and the North is pretty much just Waterdeep, with no North (a few nearby places like the Goldenfields were covered in an accompanying Dragon article). But it's a pretty good overview of the city (if a bit obsessive about the guilds). Love the map.
FR5 The Savage Frontier is the more general supplement covering the area to the north of Waterdeep. It's also pretty good, but it has a somewhat different feel than the gray box, in places. Probably because it's by Jaquays (yes, that Jaquays), not Greenwood. Covers a vast region, so not a lot of detail on any of it. One of the major focuses is the Uthgardt barbarians, and the protagonists from Salvatore's early novels also show up.
N5 Under Ilefarn is one of the best intro adventures ever produced for AD&D. It's occasionally a touch generic and some of the plot hooks are a bit railroady, but it establishes a home region for the PCs, and then introduces the players to the area with a bunch of smaller quests that take them to the major settlements and introduce many of the key NPCs. Though if you get this, you'll want to use it as your home base, not Luskan or Cormyr -- the Daggerford/Laughing Hollow region is south of Waterdeep, and a bit inland from the Sword Coast.
I still recommend Sword Coast Adventurer's Guide as it's got a lot of info, but for 1e? The 2e FR Adventures book, any of the Volo's city Guides and Aurora's Whole Realms Catalog.
I quite like Jaquay's The Savage Fontier. It's essential if you're going to run a Luskan-focused campaign, I think.
I also am quite fond of Moonshae, as the Celts-versus-Vikings character of the setting is cool. It feels a bit separated from the rest of the FR, though, but I don't think that's a bad thing.
Those are the two 1e supplements that I would use if I were to run a 1e era FR campaign (along with the Grey Box, obviously).
In terms of adventures, I think that N4 and N5 make a decent sequence. But if you're going to be focusing on Luskan, then maybe they aren't that helpful.
My favorites are Waterdeep and the North, and The Savage Frontier.