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)?
WotC has released free PDFs that provide you with some of the content from the PHB, DMG, and MM:
- Basic Rules - Apart from the core rules information, the Players' PDF gives you the most iconic D&D classes and races, and a small selection of spells. The DM's PDF gives you a small collection of monsters and magic items. They can also be viewed online - Players / DMs.
- SRD - All the races from the PHB (but only one subrace of each), and all the classes (but only one class option template for each), and a good selection of spells, monsters & magic items. There are also 3rd-party sites that lets you view (and search) the SRD online: Open5e or 5e SRD. Also, D&D Beyond combines the Basic Rules and SRD into one, but just calls it Basic Rules.
- Elemental Evil Player's Companion - All the spells referenced in the Elemental Evil adventure that weren't in the Basic rules, and a few new races.
- There are also supplements for Hoard of the Dragon Queen, Princes of the Apocalypse, and Rise of Tiamat that detail all the spells, monsters, and magic items referenced in those adventures that weren't in the Basic rules.
(I have created an index of all the above documents that you may find useful)
You may also be interested in the Unearthed Arcana articles, that detail not-fully-tested options. These may or may not appear in later books, in modified form.
There are also 3rd-party supplements on the Dungeonmasters Guild, some of which are free.
There are a few things.
- The basic rules for players and Dungeon Masters provide all the information needed to start play, but not much in the way of monster stats.
- The Systems Reference Document ("the SRD") includes a subset of the monsters from the Monster Manual and the magic items in the Dungeon Master's Guide. All of the information from the SRD can be found (and searched) on open5e, which may be more convenient than a monolithic PDF.
A site called Open5e recently launched that has everything from the SRD: rules, GM guide, character creation guide, spell list, some monsters, and equipment.
It's easily searchable and it's open source, so people can contribute more info if there's anything they've missed.
D&D Beyond is the official digital source.
For free you can access the Basic Rules; this includes races, classes, archetypes, 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.
But, D&D Beyond does make it convenient to buy additional content. You can buy digital copies of rules, supplements, and adventures 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) 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
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, there are a number of online supplements, most of which contain monsters and magic items.
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. 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.