I have a 1998 XK8 approaching 60K miles. I have read the warnings on the tensioners and it seems like a no brainer to go ahead and get the top tensioners replaced (I will have a mechanic do it).

Everyone recommends the top tensioners be done but many seem to put off the lower tensioners -

Question is:
Is it simply because it's a more difficult and costly job or are the lower tensioners less prone to cause a catastrophic failure?
Any idea how much more difficult it is to do the bottom tensioners too?

 

The bottom tensioners are less prone to failure than the upper secondary tensioners. Replacement means removing the front engine cover for access and all of the things mounted on it.
Normally, with a good scope you can check the condition of the lower tensioners. It isn't the tensioner itself that is the problem but the guide slippers they provide tension to. If they are cracked and chipped they need to be replaced.

 

Given the age of the vehicle, it is false economy to just replace the secondary tensioners and chains.

As stated above, the primary chain guides are prone to cracking and becoming deeply grooved over time eventually breaking apart and ending up in the sump.