You need as little as the battle box for your faction, it comes with Quickstart Rules (QSR), the models, and cards for the models. You also need about 4d6 and a tape measure.
The BaseRuleBook (BRB) is helpful to have and includes more detail than the QSR. Eventually you'll probably want that.
You can, but are not required to, purchase the "Forces of Warmachine" book for your chosen faction. The way it works is that every model comes with its rules on a card in the blister or box its packaged in. If you have the rulebook and buy your models off the shelf you'll have all the cards you need to play. The "Forces" book is only helpful if you want to read the fluff, get a faction painting guide, read more about individual units etc, but contains no GAMEPLAY RELEVANT information which isn't also on the cards or in the BRB
If you are starting with another person or can find a buyer for it (or if you want to start with two factions) the 2-player Battle Box is a great starting spot. It comes with a mini-rulebook (same as the Black Reach book in 40k) that is an exact reprint of the gameplay rules from the BRB but leaves out the army descriptions, fluff and painting guides. It also includes templates, $180 dollars worth of models, cards, dice I think for $90.
The game is played at any points level really, between 15 and 75 points (which would be about 250-4000 40k points). Tournaments are usually 25,35 or 50, but 80% are 35 points.
The faction battle boxes come with about 11-12 points of models and the two player box comes with 18-20 points per faction (enough to play full games).
Warmachine and Hordes are the same game. The only reason they're marketed differently is because of branding and copyright issues. The Warmachine rulebook is 95% the same as the Hordes rulebook, the only differences are in how resource allocation works, but all the combat, movement, terrain, stats and gameplay rules are the same.
The local tournament scene is very strong. There's an email list serv at Maryland Warmachine Players. It has 100+ members. Games and Stuff hosts a tournament every month with prizes etc. This month's is THIS SUNDAY, 25 points, starts at 12. Dreamwizards in Rockville has a tournament about once every 3 months, Critical hits in Bel Air is every 3 months too, there's a store in Timonium that's going to start having tournaments and there are larger national and regional events as well, including NOVA, Blob's Park, Templecon, Adepticon, Warstore Weekend, Warmachine Weekend etc.
There's no real 'meta' and there's no dominant faction in the local tournament scene. I would say there's about 50 semi-regular tournament players, of which between 12-20 show up to any given event. Tournaments are fast (6 hours typcially) decisive (we play until there's one undefeated player) and fun (there are NO rules disputes to speak of since the game is tightly written).
I'll be at GNS on Sunday running the tournament but would be happy to show you a demo game, the store has demo kits and I can run you through a small game to show you the rules and feel for the game in anywhere from 10-45 minutes.
Hope this helps!