M&B: Warband is the only version you need. It's a stand-alone version of the original, but with bug fixes and new additions, which makes me wonder why they still sell the original M&B. M&B: With Fire And Sword is a horrible, unbalanced mess. The only changes that version makes is the introduction of firearms, controllable trade caravans, and a real world setting. You can see TotalBiscuit's first impressions of the game here.
As for Warband mods, try taking a look around here. Also, Napoleonic Wars is available as a DLC for Warband if you want some sweet, sweet Napoleonic multiplayer action.
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