Let's delve like it's 1981!
We'll use D&D Basic Rulebook (B/X edited by Tom Moldvay) with some modifications from Labyrinth Lord to take a short excursion underground for no other reason than to explore and get some loot.
You do not need to own or buy D&D Basic, I've run and played in many sessions where less than 1/3 of us had the rulebook.
Besides, Labyrinth Lord is free (the no-art pdf available on their site, you can buy a print copy on Lulu) and virtually identical. If you really want to buy one, the link is to a PDF. If you want a dead tree copy, check ebay. Unless it's in pristine condition, it rarely sells for more than $15.
The differences that matter most to PCs are probably the fact that I'll follow LL's lead and let Clerics have a spell at 1st level and we'll use the Thieve's skills target numbers from LL (they correct what is a flaw in the Basic book, where thieves have a worse chance of detecting traps than any other character class).
My intent is to test the waters to see what kind of response I get and whether anyone wants to start up a monthly old school meetup.
I'll leave 20-30 mins for rolling up characters (that's all it takes in B/X), but I'll have pre-gens on hand for those who arrive late or who need replacement PCs.