I would recommend the person purchase the Mentzer Basic Set ($4.00), along with the D&D Rules Cyclopedia ($4.00) for their core rules.
As for introductory modules, the B1-B9 megamodule is a great bargain, but as previously mentioned, incomplete. I would recommend buying B1: In Search of the Unknown ($4.00) & B2: Keep on the Borderlands ($4.00); they're both short & sweet, & a good introduction to Classic D&D.
You can get all of these at
www.paizo.com (that's where I usually purchase my modules). The scans are pretty decent, & the best part is, your only spending a total of $16.00. They could spend their left over money on dice, extra modules, a standard size chessex hex grid, miniatures, etc.
They could also save $4.00 & download B3: Palace of the Silver Princess for free at
www.wizards.com, in their "Free Downloads" section.
All together, for $16.00, they received (2) classic rulesets, along with a whopping (5) adventures/modules (if you also count B3, the solo adventure in the Basic Set, & the introductory module contained therein as well).
Now that's a deal!!!