1999 Other Jaguar Models tensioner replacemet

I have purched a 1999 Jag XK8. In my research which I should have done more of, Im hearing lots about the timing chain tensior failure. As of now no problems. Should I do a preventive tensioner replacement. What would be a fair price? Also the transmisson going from drive to reverse some times does not want to take hold, and has once would not hold in park on an incline.