I'd just get Warband if I were you. It's the same as the original Mount & Blade, but better. Dunno anything about the upcoming Napoleonic Wars DLC, though, and I haven't played With Fire & Sword but I've heard it's not as good as Warband.
With fire & sword is not quite as polished as Warband, but still a very good game, and I found the setting to be more interesting. It is actually based on a book, which in turn is based on historical events, though as this is more of a sandbox-CRPG, the story is in itself not all that important.