To add to what Dathon said, you should probably download the Diplomacy mod and start it playing with that instead of Native; it expands... well, the diplomatic functions of running your own realm and otherwise interacting with other lords in a logical way that should have been implemented originally.
Other mods to think about; there is a pretty good Game of Thrones mod released (A Clash of Kings), a very tough but rewarding mod called Prophesy of Pendor (this is a good mod to use after you get good at vanilla), and a few interesting Europe mods (one that I've been following is Blue Blood, which changes the troop recruitment system). There is a huge variety though, and I'd recommend going over to the Taleworlds forum to take a look.
DLC wise, you need nothing except Mount & Blade Warband. The original Mount & Blade is not really much different other than lacking features (it possibly has a few mods that you may be interested in), while Napoleonic Wars is Multiplayer Only. With Fire and Sword similarly brings guns to a singleplayer campaign, which takes place in Europe. However, it is not highly regarded, and has received little developer support.