Do they have their own stories and whatnot (with VA and cutscenes), or are they just maps with sprites/models of the other characters?
They do have VAs in the Japanese version, but no cutscenes (if you're talking about CG).
Each chapter has its own plot. The first one we know of is Champions of Lore. There's 3 parts to that, and each one takes place on the same map, but different times of the day. I believe all chapters under that title follow one narrative, though I can't couldn't read the text in the Japanese version. Likewise, there are also 3 other unique maps, with 3 pieces of dlc each with the same stage being slightly different in appearance for each part.
That's just the retro maps though (so 4 unique maps, but a total of 12).
Without going into spoilers, there's also 3 story chapters related to the main campaign, that take place after. You'll know what I mean once you beat the game.
The map where you can obtain the sacred treasures also has a light narrative, with a couple villains from the first game.
Then there's the hot springs and another one I'm forgetting which has about 200 support convo's when you play them.
Even 6 of the spotpass chapters have a plot, usually surrounding non-playable campaign characters that you can recruit by playing, however these were release bi-weekly in Japan, so it took 12 weeks to get all 6.
Idk if I'm just remembering wrong, but did they say something about offering DLC initially for free or something?
The first DLC chapter, which nets you Marth, is free for a week I think.