The King's Bounty series is great. But it is also quite different from the HoMM series. In HoMM you have to upgrade your cities to get better units and level up your heroes so that they are great army leaders which allow you to conquer the whole map. HoMM is strategy with tactical battles and heroes development.

in King's Bounty you have only one hero/heroine. You get to explore the map, collect treasures and buy new units for your army at certain places. There are also lots of quests. The focus is more on adventure/rpg with lots of tactical battles, but nothing to build.

I would agree with Tallima on the raniking of the heroes games.I also started with the first part of the series and played all of them (excluding part 6 because of steam). III is absolutely great though I have a soft spot for II.

As the first game to introduce yourself to the series I would recommend Heroes of Might and Magic 2, one of the best games ever made. Then HoMM 3, and ony after that 4 and 5.

If you don't afford to buy all of them then I recommend you Heroes 3.