I'm a little confuse if you mean is 64K or 120K high miles for a V12 -
Either way its not, the V12 engines are indestructible. Problems seem to be
centred around oil leaks, so have a good check around to make sure its nice
and dry. Make sure that the oil pressure remains reasonable when the engine
is hot - Figures vary but look for 20 psi or more at idle.

Assuming you will have a good check around at the generic car rust spots -
Sills etc, then these are the places to pay particular attention on an
XJS...

First place to go to is the inside of the wheelarches on the front and rear,
the big tyres throw stones up an can easily crack the underseal. Pay
particular attention to any corrosion on the bottom edge of the front wings.
If its anything more than just surface rust then you will be looking at
welding work to the sill behind - Again a really common fault.

Open the bonnet and check on the inside of the inner wings where the shock
absorbers come through, just below the shock absorber nuts is another common
corrosion point.

Jack the car up and make a really good inspection of the rubber bushes where
the lower front wishbone mounts onto the front subframe. Replacing these on
the V12 model means dropping the front subframe which is not an expensive
job buts its really fun. While your under there have a good check at the
subframe to make sure its not rusted through.

A good trick to check on the rear suspension bushes is to get a glamorous
assistant to sit in the car with the engine at idle, handbrake off an
footbrake applied. Put the car in D and watch the rear wheels. There should
be no movement of the wheels forward, do the same in R and again there
should be no movement backwards. Again, these bushes are not expensive to
replace.

Essentially parts for XJS's are quite cheap thesedays and are really easy to
get at places like David Manners of Birmingham - Its just some of the jobs
are a bit more fun than you might have with a Ford Escort.

Oh, and a word of warning - There are certain members of this news group
that would have you believe that the V12 engine is not the
be-all-and-end-all of Jag engines - Of course these people may point out
that there are newer Jag engines which generate more BHP, but the V12 is on
a different planet when it comes to delivery of the power. Dont believe a
word of it - Go out there and get that V12

Best wishes

Stu