Red Dead Redemption is by far my favourite game of the year. That said, I've had zero interest (ie: friends) in playing the game online.  
Is all this zombie stuff not for me or is it offline as well?

It ties into the story so it's offline. All the videos you see for the Zombie DLC are part of the single player.

As far as I know everything they've shown up to this point is singleplayer. There's supposed to be an online component to it as well, but it's unclear whether it'll be the whole thing in co-op, a horde mode, or just multiplayer skins.

" As far as I know everything they've shown up to this point is singleplayer. There's supposed to be an online component to it as well, but it's unclear whether it'll be the whole thing in co-op, a horde mode, or just multiplayer skins. "

The main focus of Undead Nightmare is a new single player campaign.

The first two paid DLC packs were almost only aimed at the multiplayer. The new(er) Undead Nightmare DLC is mainly focused at the single player. It might be as big at one of the GTA4 episodes. It also has some multiplayer skins... 
 
From what I undertood (when ordering it) the seperate DLC Disc, you get all 3 DLC packs. I don't know the real retail price, but I found it at 20$ at the canadian Amazon (also that price at HMV). The disc is a standalon product like the GTA4: Episodes form Liberty City disc.