I see it has 7 (games?) to the series, correct? Or are some of them DLC's? If so, which are the actual games and which are the actual DLC's? Or are all 7 standalones and not necessarily connected?
In order of original releases:
King's Bounty the Legend = complete game (no DLC available)
King's Bounty Crossroads = original Armored Princess game + DLC
- Armored Princess takes place after events in The Legend.
King's Bounty Warriors of the North Premium Edition = original Warriors of the North + DLC/Premium Upgrade
- Warriors of the North takes place after events in Armored Princess, but in a different part of the world
King's Bounty Darkside Premium Edition = original King's Bounty Darkside + DLC/Premium Upgrade
- After beating up on dark forces in the first three games, this one lets you be on the dark side beating up forces of light.
You can buy the premium editions (on sale if possible), or buy the original editions + premium upgrades (later on).
If you get Crossroads, be aware the Orcs campaign is the same as Armored Princess campaign + extra quests and orc enemies.
DLC/"Premium" content typically includes additional maps, quests, challenge campaigns, and/or enemy units, depending on the game.
Hope that helps.