Microsoft keyboards claim the longest battery life. Well, according to the box thats what they say. But I use cheapy batteries and I can usually go about 2 to 3 months without replacing them. Which I think is pretty darn good, I got a box of 80 AA batteries for $10, these are REALLY cheap.
The bluetooth sets come with a receiver, I mean they should anyway. But like jlockley said use someplace with a return policy so if you aren't happy with it you can return it or exchange it.
Cheers
Rob