Gathering Storm probably depends a bit on the group and what kind of characters they make. The WFRP scenarios tend to be written very much for wandering "adventurers". Which does not really fit many gaming groups and the kind of characters they have made.
Personally, I prefer to make my own campaigns so I can just make stuff up if I have to without fearing it might contradict something in the adventure. And I can tailor it much more to fit with the characters that are being played.
After the Core box, I would say the first thing to get is the Adventurers Toolkit. This fills out the selection of careers and action cards (It's a bit thin with just the core set). Personally I think the Adventurers Toolkit should have been part of the core set. It's like they carved out 1/3 of the Core set and sold it in a separate box.
The Guides and Vaults are generally not needed. They are just reprints of what you already get in the proper boxes. Although the players can benefit from a players guide to plan their characters etc without borrowing all your cards and stuff.
For other expansions I would honestly take a look at what careers your players chose and what kind of emphasis you want on your adventure.
Winds Of Magic: More wizard careers, spells and wizard orders. Also, rules for mutations and Tzeentch, the god of manipulation, magic and mutation.
Signs of Faith: More priestly careers, invocations and faiths. Also, rules for dieases and Nurgle, the god of pestilence, plague and decay.
Omens of War: More warrior careers, combat action cards and mounts. Also, rules for permanent injuries and Khorne, the god of war, carnage and bloodshed.
Lure of Power: More noble careers, social action cards and followers(I think that was here?). Also, rules for "social battles" and Slaanesh, the seductive god of pleasure and pain.
In time, you should have all four of these (although you can hold off on Lure Of Power. Personally I feel it brings less to the table than the others unless you're running a campaign set in high society) as they add very iconic mechanics to the game.
Edited by Ralzar, 12 October 2013 - 11:10 AM.