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: 
[D&D Beyond](https://www.dndbeyond.com/) is the official digital source.
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For free you can access the [Basic Rules](https://www.dndbeyond.com/sources/basic-rules); this includes [races](https://www.dndbeyond.com/sources/basic-rules/races), [classes](https://www.dndbeyond.com/sources/basic-rules/classes), archetypes, [monsters](https://www.dndbeyond.com/sources/basic-rules/monsters), Unearthed Arcana, etc, everything you need to run a game. The site also contains useful features, such as character builders, and campaign organisation tools. Everything is searchable, linkable, and well organised.

I would go so far as to say that most players probably use D&D Beyond for its tools and rules referencing, even if they own the books in some other form.

Anything else that costs money, will still cost money to access.
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But, D&D Beyond does make it convenient to buy additional content. [You can buy digital copies of rules, supplements, and adventures](https://www.dndbeyond.com/sources) one at a time or in packs too.

Most items, classes, and monsters can be purchased individually at an even lower rate. Say you want the Abyssal Chicken, instead of buying the source book it appears in ([Baldur's Gate: Descent into Avernus](https://www.dndbeyond.com/marketplace/source/baldurs-gate-descent-into-avernus)) for $30, you can buy all 38 monsters in that book for $5, or just the chicken for $2.

Note that the Basic Rules include the SRD
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The Basic Rules contain the entire SRD, so there is no need to worry about it:

> 
> This section contains the Dungeons & Dragons Basic Rules and the rules in the SRD, released as part of the Open Gaming License.
> 
> 
>
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).