If you want the basic rules built in, your two best purchases are likely the Players Handbook, and the Monster Manual, which can be ordered in bundles form the Marketplace. That's enough to run a good solid campaign with. There's lots of expansions you could buy, but those are the core rulebooks. The Dungeonmaster's Guide is not essential, IMO. It might be cheaper just to buy those two separately. The SRD (System Reference Document, or the free content) is what Wizards of the Coast provides as introductory material to the game, and as a basis for creating other, marketable content without the need to negotiate a license. That is the content that is freely available on Roll20. Although limited, you can run a basic game with it. And yes, even at the free level, you can share your content with one game of up to five players. More games and players are available at other subscription levels: Compendium Sharing Edit: Be sure to claim Lost Mine of Phandelver before the end of the month. It's free until then, but if you create a game with it before then, you can continue playing it. (You could even make multiple games with it if you want). It has a lot of monsters with art, some great maps and a good introductory adventure.
