Beware of the v12 , used till around 1995 or so in the xjs model, and the xj12 sedan. They are great motors when maintained, when they are neglected they have issues with low oil pressure and running hot. The v12 uses a TH400 GM trans. from 1977 onwards. Most models have inboard rear disk brakes, if the pads go to metal, the labor to replace the rotors is quite high.

In short, a ppi is very important, as are complete service records.