Glad to be hereBut now I've got the urge to buy more books so I can run something old myself! :O
I've got good news for you, then. AD&D didn't have the explosion of supplemental books that really started with AD&D 2E then went super nova with 3E. For AD&D, you've basically got the DMG, the PHB, and the Monster Manual.
All of our heads popped off when Unearthed Arcana came out. And, if I'm not missing anything, there were only six or seven other books in the entire AD&D line-up: Monster Manual II, Fiend Folio, Deities & Demigods, Dungeoneer's Survival Guide and the Wilderness Survival Guide. Oh, there are the world books: Greyhawk Adventures, Dragonlance Adventures, and Oriental Adventures. I think that was it. Maybe I'm missing a book or two.
So, what do you need out of that? The DMG, PHB, and probably all of the monster books: MM, MMII, FF. The other stuff you cherry pick for your game.
Here's the other good news: If you shop around a bit, you can find all of the books for cheap, cheap, cheap, dirt cheap.
Now, if you just do a Google, then you're playing something like $66 bucks for the Player's Manual.
Don't do that.
Hit your local used books stores and look for deals on the net. As I wrote this, I found the MMII for $8 bucks and the DMG for $13 bucks on the net in about 30 seconds. Last year, I ran a short AD&D game that lasted about six sessions, so I hit the local Half Price book shops. I found both the PHB and the DMG for under $9 each. In real good condition, too.
So, you have few books to buy, and if you're a smart shopper, you'll get them at rock bottom prices.
Figure about $30 bucks for the PHB, DMG, and MM, and you're off to the races.