1) How likely are they to reject, as far as the ranking of most -> least likely for surface piercings?
Napes are generally considered the most 'stable' of surface piercings. Sternum piercings can be pretty long-term, depending on your anatomy (people with bigger boobs tend to find them more awkward to heal - more pressure when you're not wearing a bra, etc) and the jewellery.
2) Where would the skin need to be pierced to make it less likely that it is rejected? I've seen them in so many different places it confuses me.
The piercer should position it where it's most suitable. Try putting 2 dots an inch or so apart where you'd like it placed and stand in front of a mirror moving your arms around and stuff. Yeah, you'll look like an idiot, but basically the positions where the dots move around the least will be the best ;D
3) How long do they typically last? Can you repierce easily? Is it recommended?
They can be as long-term as any other piercing or they can last a week. Just please make sure you go to an experienced piercer and if possible ask to see a portfolio of both healed AND fresh surface piercings. Make sure they know what they're on about. That means using an internally threaded titanium surface bar that's actually measured for you. You might need to go in before for an appointment where they'll figure out what jewellery is best and order it in.
4) How much do they cost?
Probably £30-£40 depending on the area and the piercer.
Where abouts do you live? Someone can probably recommend a good piercer.
Also, I strongly suggest using disc ends rather than balls, at least for the initial healing period.