1st Edition books...

[Akrasia]

The three core books are all you need. UA has some good stuff, but use with caution. There is enough cool stuff in MM2 and FF to justify their purchase (probably $5-10 each on ebay). DDG is purely optional -- I would just buy the pdf (at most).

The modules were perhaps one of the greatest thing about OAD&D.

I am sure lots of people will mention certain well-known classics like the G/D/Q series, or the S 'killer' modules.

But I want to strongly recommend you pick some underdogs (especially if you are starting a new campaign).
N1 is a great intro module, and L1-2 are also excellent.
But the UK series (especially UK2-4, but the others are wonderful as well) is perhaps the best OAD&D series. UK4 blows me away to this day. They are great adventures with great plots -- but no horrible railroading (unlike the annoying Dragonlance mods).

 

[Ferox4]

You can't have my books, you can't have my books. No! No!

Get the 3 core books and have at. There are modules galore out there to be had for a song. And don't forget to check the potion miscibility table if a PC drinks more than one potion.... ..... ah, 1977, who is this middle-aged man impostering me?

 

[Henry]

Also, don't forget that almost ALL of the 1st edition books are available in PDF on www.rpgnow.com , if you're a fan of that format. Even if you prefer hardcover, they are useful for searchability and cross-reference.

 

[jasper]

What you may want to do once you get the monster manual. Is take a pencil and copy the xp amount from the dmg to monster entry. I think the Fiend Folio did it first.

 

[Voadam]

at www.rpgnow.com you can get them each as a pdf for about $5 or $6 each.

The 1e DMG has a chart with all the monster statistics from the MM, you just won't have any pictures or flavor text for them.