Kingdoms is fantastic, especially if you love a specific area of medieval history (I fell in love with the Baltic crusades after playing the Teutonic campaign) and the mods for Kingdoms are on par with or better than those for Medieval II.
Buy kingdoms. Play it. Get Stainless Steel (possibly the best mod I've ever played). NAO

IIRC, Kingdoms doesn't really affect Medieval II. It just installs its own campaigns with new features and such and doesn't really alter the original. Your Medieval II mods should work fine with Kingdoms.