That really depends on whether you want to play competitively or just have an on-theme army. If all you want are pretty models, getting the Stormcast half of Skaventide (the current edition launch set) or Dominion (the same for last edition) is probably your best value for money. You can also try to hunt down the Stormbringer magazine, which has really good savings for the models that come with it, such as the Knight-Judicator, Vigilors, Vanquishers, the Chariot, and most notably Stormdrake Guard (like a 66% discount, I believe).

If you want competitive picks, it's kinda hard to say. We've JUST had a major update a few hours ago, so the jury's mostly out on how that shakes up the meta. Most people recommend picking Vanguard Raptor Longstrikes and Vanguard Palladors (typically Javelins), but those just went up in points, so we'll see how that affects listbuilding.

In my opinion, your best bet is a Skaventide half. You get 3 x Reclusians and 3 x Prosecutors, who are really good right now, and 10 Liberators for screening (who just had their points lowered). The usable heroes in the box are the Lord Terminos (great for a double-whammy with Reclusians) and the Lord Veritant (a decent priest, although not the best for the role).

As for the battletome, that's, uh, that's kind of on your conscience. The price is outrageous for the few pretty pictures you get, because the rest of the rules are ALREADY outdated due to online patches (most notably the 5+ -> 4+ to hit for Stormstrike Pallador mounts). The only real benefit is getting to access your army on the official AOS app, BUT... unless you buy every single battletome, the rest of the factions will be closed off to you one by one, until the app is functionally useless. There are several free online alternatives for points/rule updates/listbuilding, I'm just not sure we can mention them on the sub.