Here are some original old school D&D modules that should be easy to find and not cost you an arm or a leg if you buy them online:
I1 The Forbidden City (TSR, 1980): Old school AD&D sandbox setting in a "lost" jungle city inhabited by snakemen (yuan ti), frogmen (bullywugs), and lots of other weird and dangerous creatures. Lots of room for development here and no two adventures will play alike (due to multiple adventure paths and foes).
B4 The Lost City (TSR): This old school original D&D module consists of a pyramid buried in the desert sands which holds various degenerate groups battling each other for control of the underground empire.
X1 Isle of Dread (TSR): Lost island sandbox setting with lots of different adventure ideas
WG4 Lost Temple of Tharizdun (TSR): written by the master himself, EGG, this AD&D adventure takes place in a long abandoned temple to a dark god hidden in a mountain valley. Initial conflict is against monsters who now inhabit the structure, but if the party delves deep enough they will run into Things Best Left Undisturbed.....!
Axe of the Dwarvish Lords (TSR): This AD&D supermodule written in the 90s takes place in an long abandoned Dwarvish citadel now inhabited by an army of goblins.
Gates of Firestorm Peak (TSR): This 90s AD&D module takes place in a weird, otherworldly mountain that has connections to a "Far Realm" of madness and insanity. Sounds like it would fit right in with Raggi's stuff!
Not as easy to acquire are the classic Judges Guild modules Caverns of Thracia and Dark Tower (however, the 3.5 reprints should be a lot easier to find and are backwards compatible). Both are great and concern ancient, underground empires with lots of evil things lurking about.
D&D set the mode of play for RPG's through at least 1980... certain others deviate from the mode of play into more abstract in the 80's, and into more story driven in the 90's.
I'm rather certain that Greg was aware of RPGs before he bought a copy of D&D, as he was a game designer, and had seen play at conventions. (He's mentioned this in some discussions on WWG's Pendragon forums, now defunct.) When he wrote RQ with Steve Perrin, there were already at least 3 games on the market: D&D, Starfaring, and T&T. And Starfaring and T&T are in fact responses to D&D by Ken St. Andre.
Greg has not mentioned participation in the Braunstein games (which predate D&D by several years, and are part of the origin of D&D).
The quote in the question is the proof that D&D influenced RQ... Greg had a copy, found it editorially lacking, and set out to do better.
Note: RPGGeek cites Greg Stafford and Steve Perrin codesgined RQ1, and it was published in 1978; Boardgame Geek notes that White Bear and Red Moon was published in 1975. That frames the timeframe for D&D influence on RQ1.
T&T was 1975, as was Starfaring. Both by Ken St. Andre.
Best Answer
If you look at
D&D Archive.
It seems to be that Palace of the Vampire Queen was the first somehow published Adventure in 1976.
Here are some more details about it: It's the first D&D Module ever published, but not the first TSR one. There was also a scenario - The Temple of the Frog - that was published earlier in the Blackmoor supplement (pages 27 - 47).