@jdrakeh
I'd suggest the following:
* Either PHB 1 or PHB 2 - probably the first if you want the most "classic" classes and races.
* The DMG and DMG 2 and Rules Compendium (the last is an Essentials book) - the RC will give you all the rules for action resolution and the final and best set of DCs-by-level; DMG 2 is good for skill challenges in conjunction with RC; DMG is also good for skill challenges and also for combat encounter design. I don't know what the price tags are on DriveThru but if that's too much, probably drop one of the DMGs (DMG 1 has got the treasure parcel tables, the version of these in the RC is not as good in my view (too random) but not terrible; the treatment of Quests in the DMG is better than in the RC, though to get the fullest (still only about half-a-page) discussion of player-authored Quests you need the PHB 1).
* The Monster Vault (an Essentials book) that has most of the classic monsters with MM3-quality stats. It's a bit thin on Epic tier creatures but if that turns into a problem for you you've been playing enough that you can get a bit more stuff!
I know it's a bit weird to be recommending more "how to do it" books (DMG, DMG2, RC) than books-full-of-stuff (PHB, MM/MV) - but to get the best out of 4e (in my view) you need to combine a number of techniques/approaches that are scattered across multiple sources and are a bit of a departure from classic D&D. The more you're familiar with "indie"-ish games (especially scene-framed ones like HeroWars/Quest or Burning Wheel or Cortex+) then I think the easier you'll find it to do well with 4e, in which case mabye drop the DMG 2 but post to get its advice on how to treat powers, surges etc as a uniform currency for skill challenge resolution.