Well, BG is AD&D 2nd Edition, without all PHB options (like no non-weapon proficiencies), some rules additions from sourcebooks (like the kits from the "complete" classes series), disregarding other rules from the same sources (like no ranger/bards and bard/druids from the complete bards), and introducing some stuff from 3rd edition like sorcerers and monks (though some of that actually is based on 1st Ed). Basically it's some 2.5 Edition.
Well, I have a huge shelf of old AD&D books as well as 3rd and 3.5 Edition books (ahem, and Pathfinder stuff), as well as some CDs like Core Rules + Evermore Expansion or FR Atlas, along with loads of PDFs on my HD (which is completely ok since I DO own the genuine paper originals), but, this might not apply for many other players.
So what P&P rules IS BG:EE actually based on? Since 2nd Ed is not published anymore for quite some time, is there a chance that Wizards of the Coast might allow the devs to publish the rules that are supposed to be the backbone of the game - at least for the betatesters to be? After all you can't betatest rule behaviour if you don't know what the rules are supposed to be.