Random thoughts on the matter:
For me, the answer is mostly "don't buy a V12 XJS" because they do have a *lot* of problems. The 5.3L V12 had several problems involving dropped valve seats (which immediately wreck the engine and require a rebuild) if they're run the least bit hot. Another endearing feature was the 90-92 cars' tendency to have catalytic converter fires because the Italian ignition system would temporarily fail on one bank, then start back up after the cat was full of gas. Earlier cars can have engine fires due to the fuel injector hoses getting brittle and leaking due to the heat in the Vee of the engine. ALL of them can have distributor explosions due to how they're arranged.
The 6.0L V12 introduced in 1993 is the best of the lot. They fixed most of the problems, and if you find a 96 (last year for it - though technically the last ones are leftover 95s) it *may* have a Ford distributorless ignition (or you can steal one from a dead 96 XJ12). The 6.0L cars also have the very latest equipment and electronics fitments for the XJS, and those are very reliable. In addition, they also got the 4L80E transmission (4 speed overdrive unit from GM) so they get a little better gas mileage. They also have conventional outboard brakes, so maintenance is easier.
So, if you MUST get a V12, get a 93-96.
That said, you will find that the XJS is more of a handful to own compared to the Jaguar sedans. The heat of the engine tends to mean more regular hose, belt, and component changes in the engine bay. Since the XJS was constantly revised over its entire lifespan, body parts often do not interchange across model years and therefore are more expensive. Electronics are usually shared with the same year or close to same year sedans; there is a tiny but dedicated aftermarket, and you can even get a Tremec 3550 or TKO manual transmission conversion (US XJS was never offered with a stick.)
There are also a few things you can do to get a little more power out of the beasts without too much money - the big one is fitting free-flow cats, removing a couple of restrictions in the exhaust, and going from four mufflers to two. This is typically good for 20hp or so over the original 322hp (in 6L cars).
As is the case with any Jaguar, you want to look for one with a FULL documented service history. As someone noted above, you want one with NO rust, so check under carpets, in the trunk, and under the car for rust spots. The only point where rust is actually *fatal* to the car is the radius arm mount points in the rear (assuming that you have full rust-through in spots), but rust *anywhere* can be expensive to fix and would need to be remedied immediately.
If your father is willing to pull much of his own work, the XJS is relatively easy to work on. There is a lot of good documentation out there; if you pull your own maintenance, the cost of ownership is surprisingly low. There are also a large number of mail order parts vendors out there with *great* discounts on parts. And, unless you have a really bad dealer, most parts for these cars are usually less than comparable parts for a Mercedes, BMW, etc., etc.
Finally, and I know this seems counterintuitive, don't automatically go for the car with the lowest miles. Jaguars like to be driven; ones that are driven regularly have a lot less problems with seal leakage and parts just dying for no apparent reason. Ones that sit around and then are sold as "low mileage" examples tend to end up being giant money pits. A properly maintained, regularly driven Jag is a marvel, a wonderful car that will serve with few problems for many, many years. A rarely driven, sporadically maintained Jag will be an unforgiving bitch that will eat all the money you have and ask for more.
If you would like, I'll see if I can get my XJS expert friend in here - he's more likely to be able to give you an in-depth run-down by year, if you want.