For Stormcast its pretty easy
1) The Battletome - as said its new and thus pretty much up to date. Grab the Errata and FAQ off the website and you're good to go there with all your core army options present in the book.
2) The Big Rulebook has a few extra missions and the detailed rules for matched play - which isn't that much but has a few key bits - like you can only use 1 endless spell of any name at a time (whilst the endless spell rule page would suggest you can use as many as you have models for). You can, of course, use as many different endless spells as you want. It also has the rules for using different Realms for your army and for the battlemaps.
There's also loads of lore and art and other stuff, but in general to get started you don't need it.
Again get the Errata and FAQ
3) The Malign Sorcery boxed set is worth getting; it gives you one of every generic endless spell to use (barring Balewind Vortex) as well as the rules (on cards) to use them. In addition the book has expanded equipment and spells for the realm rules system.
Again Errata and FAQ on the website
3) The Generals handbook has a few extra battleplans you can use and some updated points, but honestly as Stormcast are so new I don't think there are any updated points. Thus there is nothing (besides Battleplans) in the Generals Handbook for them so you don't "need" it.
Realm system in general is split into two parts, the map you play on and the realm your army comes from. Both aspects are optional in any game and some players use them and some don't - you just agree with your opponent before the game. As you are getting started leaving them out would be perfectly accepted by most players.
Endless Spells are also optional, though more generally universally accepted; but again you can agree with your opponent on their use.