Liquidsabre
Explorer
SO does Kingmaker really having anything in it, mechanics-wise, that couldn't possibly be converted to another system? Is it just a matter of changing encounters around to fit into your system of choice? Are there any skills, items, special rules that absolutely require use of Pathfinder or 3E? I have to admit I regret not keeping up my Adventure Path subscription but at the time I just didn't have the money and now they have so many (35? 36? books - yikes!) it seems daunting to try and decide which one(s) to pick up.
The Kingdom Building rules itself aren't at all tied to the game system at all. In fact, there is even a pdf release of just the kingdom building rules that can be found here.
No need to fret over the number of books. Since each AP is only 6 books you can pick them up in small groupings per AP. Each AP has their own summaries/themes that you can choose from. Myself I picked up Rise of the Rune Lords, Second Darkness, and Kingmaker when I decided to break into the APs and I didn't worry about any of the others as they didn't interest me yet. In fact, I've coordinated with my game group of GMs and everyone is picking out their favorite APs to run so I have no need to pick them all up except to collect them and to read the non-adventure sections.
If you are having trouble deciding (understandable that) you can read more about folk's experiences running/playing the various APs on the Paizo boards. There is a forum for each AP as well as general discussions of folk's favorite APs! Besides, missing out on the APs makes for some most excellent Amazon orders at discount. That's how I originally got mine (except for Kingmaker)!