I lusted after an XJS for a long time. I mean, powerful and smooth V12 engine, easy to service GM transmission, what's not to like? Right?
I even bought a later car with the HE heads... more power, better economy.
You know what? I absolutely hated the car. It was poorly assembled and difficult to work on, but the worst part was the horrible mismatch between the low-torque V12, the power-sapping TH400, and the high gearing. A nice car for the interstate, but a real dud to drive otherwise.
I understand one of the aftermarket suppliers came out with a cam kit that dropped nearly two seconds off the 0-60 time, but did so by moving the power curve down and losing something like 20% off the top... if true, absolutely proving the drivetrain mismatch.
Rear brakes problems can also be expensive... easiest to drop the rear subframe than to work around it on the car...
I would only consider one with a manual transmission (only sold here after Ford got involved and only with a 6, not a 12), a V12 car that I wouldn't mind converting to a manual trans, or one of the last 6.0 V12's which apparently had a much better synched up drivetrain (though I've never driven one "in anger")
Where does the future lie for these? I see the average cars with the average problems and the deferred maintenance continue to get cheaper and cheaper... much like Jensen interceptors have... but the really good cars (especially the convertibles, the manual trans cars, and the last of the line 6L's continue to hold their values fairly well)
I sold my not rusty, decent paint, nice interior, good running and driving XJS HE for $3500. And was happy to see it go.
Good luck!
Bill