My big question is how closely will this lead up to the original film (basically how prequelly will this be)?

Which sort of goes with my other question which is what will be the time span?

Since this is a spin-off, it has the liberty to jump around the timeline (time gaps, flashbacks, etc.). I could easily see this just being a film about the week before ANH but we could also get some backstory on the characters and the rebellion (for instance it's not surprise they're at Yavin, but will we see how they get there?).

Presumably the final scene could be Vader's Star Destroyer chasing the Tantive, but will they go that far? Will they show Vader? Though they could do either or both, my guess is neither. This is supposed to be a standalone film and I'd imagine they mean that in the truest sense.