Question: I'm not a big fan of spending 100ish dollars to buy all three core rulebooks. Are there monster stats to be found online? Also, is there an online substitute for the DMG (for items)?

Answer: 
As well as the free [Basic Rules](http://dnd.wizards.com/articles/features/basicrules), there are a number of online supplements, most of which contain monsters and magic items.

* [Hoard of the Dragon Queen](https://dnd.wizards.com/products/tabletop-games/rpg-products/hoard-dragon-queen)
* [The Rise of Tiamat](http://dnd.wizards.com/products/tabletop-games/rpg-products/rise-tiamat)
* [Princes of the Apocalypse](http://dnd.wizards.com/products/tabletop-games/rpg-products/princes-apocalypse)

I remember a thread a year or so ago where one of Wizard's staff said you can play the game by buying one book. I believe they suggested the Monster Manual. That is, use the free Basic Rules and the purchased MM. The idea was that reading the monster stats gave lots of cool ideas for adventures.

Personally, I think that buying the PHB is a no-brainer. The Basic rules only give you a few classes and archetypes; the PHB gives you all of them. Add the free Elemental Evil Player's Companion for a few more races and spells.

You may want to borrow the Sword Coast Adventurer's Guide before you read it. I'm hearing mixed reviews, and more than a few people are telling me not to waste money on it. YMMV.

I would also suggest [D&D Classics](http://www.dmsguild.com/browse.php?filters=45471_0_0_0_0_0). A few dollars there will get you old d&d adventures that work very well in 5th edition. I've found that 1st ed modules need very little conversion (just picking appropriate monsters for the party level).
You can also look to Fantasy Grounds (www.fantasygrounds.net). It is an online campaign manager for D&D, and other game systems. It has the rulebooks built into its libraries, and allows the creation of all the hard cover printed books. You also have options to create your classes/abilities/etc.