Are all prosumer DV cameras about the same lifespan? How long? What is the part that tends to go first? Heads? Do all major mfrs offer a refurbishing at about the same price? How much do these cost, and how often do they need to be done? What is expected total lifespan of a 3CCD cam? Will these numbers be better or worse for HDV ?

I'd say heads are the first to go indeed. I know Canon can replace
these. Another option would be to record direct-to-disk to avoid
the tape mechanism all together.

Can't answer the rest of your questions though...