Ohohoho. Warband.
First off, I would say download the diplomacy mod. It's basically native but with a few extensions (few more units, items, ability to bribe lords to not attack you or to persuade King to declare war etc) I personally use the Clash of Kings (Game of Thrones mod) which has it already included which is an entire overall. Native gets boring fast, but it depends on what your character and your army preference. If you like an entire horse army, Khergits. Infantry, nords. Heavy cav, Sarranids and Swadia (Sarranids are better) or if you like defensive infantry, go for Rhodoks.
I'd personally recommend you to go horse archer and go with the Khergits. They're pretty bad in castles, at least so I've heard. Whenever I play with them they're alright against their neighboring factions like Sarranids and the Vaegirs. If you have an entire army of Khergits you will pretty much win A LOT in the open field as long as it isn't hilly. I personally use a large majority of my army being horse archers and a small percentage of lancers. (E.g: 100 men - 70 Veteran Horse Archers - 30 Lancers) Horse archers are so overpowered, specially against the bad A.I. At about 7 power draw and a master nomad bow or khergit bow, you can pretty much one shot kill anything. Just for party skills, use party main members like Lezalit and others. If they're clearly warrior classes just level their str up. (If you want to start your own kingdom, you can make them lords! But see if their royalty first, lords dont like making a low-born a lord)