hi there and welcome
you should make sure it has a gradeing sheet and i reckon it is due its next service,
the speedo conversion should be done as part of the sva and should now clock up in miles.
sounds to me as though this guy has imported it his self cause if a company had done it the speedo would have been done.
depending on how much you are getting it for and how much you could get the same car from the dealer for would make my mind up.
i have had no experiance on the clicking axle and not sure how much it wouild cost to fix , just another thing to think about.
sorry to be a little negative but i did buy an import of someone and got caught out with a few things.
hope this helps and dosnt put you off the Z as it is awesome !!!