If you go with Hordes you can get away with just the cards and Primal, as Primal lists all the new moves.
If you go WM, all you really NEED (as others have said) is Prime, plus buy the "Deck" of cards for all your faction unit choices. The 'deck' includes everything from Prime and Escallation.
That said, here is my brief book review from the WM line:
- Primal: Must buy, all core rules plus core units from all 4 factions. This book is always in my army bag for quick refrence.
- Escallation: (Fanboy Only), new rules are available online, FAQ available online. All units introduced are available in decks of cards. Does have cool fluff. To date, I still don't own this one, although I think about it a lot.
- Apotheosis: Recommend strongly, awesome artwork, full color, new rules, great campaign rules. Can get away without, but very nice. I own this one and love it, but I don't bring it to my weekly games.
- Superiority: If you only get one suplement, get this one. The Merc Contracts are requried to understand how they work, and until Superiority the rules for these where only available in out of print NQ#1. Cav rules are also a must own. In addition the amount of detail fleshed out for each faction will help new players even further to make a choice. I've got the hard cover edition of this one, and consider it one of the coolest books I own.
In short, if I were just starting out, I'd buy Primal and Superiority. The only downside to this is the fluff "story" which is a giant arc spanning all 4 books. If you get into the background of the game, someday you'll want them all. Which is clearly PP's evil plot. >