When first introduced back in the '70s , the Jaguar XJS was a big dissappointment. People were expecting a successor to the E-type and the XJS simply wasn't it. The quirky rear 'buttresses' - an add-on to the original design - looked odd, there was no convertible, and build quality varied between fair and awful.
But things change with time. The design has worn much better over the years than many of its contempories. If you accept it for what it is - a fast sports-touring car, not an outright sports car - then it does the business. Build quality steadily improved, especially with the post-1993 'facelift' cars, (easily recognisable by their new-shape 'smoked glass' rear lights), which had galvanised bodies to retard rust. (Ignore anyone who tells you these cars don't rust, but they rust much slower than their predescessors). You can now have a convertible - early ones had a sort of fixed roll-bar arrangement and were ugly, later ones are smooth and sleek. And good examples can be had for very reasonable money. So - a good buy?
Well - hmmm. Firstly, my advice is ignore the V12 unless you are an engineer, or the purity of engineering appeals to you. It's a great engine - but expensive to maintain, and very thirsty - think maybe 12-15mpg. And has little advantage over the 4.0Lt offered in the facelift cars, which will give you double that mpg.
And as these cars age, you need to buy a good one. Even on the later ones, it's likely the sills will need replacing, unless already done, but you can buy these new very cheaply. The rear section of the front wings rusts for the fun of it, and repair panels have never been made. Only real solution is find a new wing - which you can sometimes pick up, expect to pay about £100-£120; A used one is likely to have the same problems you're trying to overcome. Floors rust too, and are an expensive fix. And the electrics will gradually drive you mad as one thing after another decides to not work, or work in its own sweet way. Central locking in particular gradually fails.
Many parts are obtainable new and very cheap from specialist suppliers, but some things will cost more than you think. Crack a rear light lens on a facelift model and you've a big problem; it will fail an MOT, there are no new parts available, and nobody will sell you a used one for less than £100, that's if you can find one. Oh and need to change a headlight bulb? Ignore the manual, the way they say to do it is impossible. You will need to dismantle the headlight.
However the engines and gearboxes are pretty much bullet-proof. Classic car insurance is uber-cheap because as they were introduced back in the '70s they count as an 'over 25 year old' car to insurance companies, even if you have a 1993/4 model. Expect to pay about £200 for a 7000-mile-a-year policy, though some people claim cheaper and may be true.
The real joy is in driving these cars. Don't as I say buy one of you want a sports car. But if you're considering say a bigger Mercedes Coupe or something similar, these cars give you all the refinement, comfort and performance in a much sportier package, and you can drive 1000 miles and get out as if you've just been asleep in an armchair for the day. But don't put anybody bigger than a 5-year-old in the back seats. They are truly tiny. Though the boot is huge.
So to summarise; only buy a really good one, with genuine Jaguar FSH. Go for the 4.0 facelift - 1992 onwards. Check those sills, front wings and floor. Expect it to cost you say £1000-£1500 a year in 'fixing things' ; not much compared to the depreciation you'd have on a newer car of half the quality but if you can't afford that, need your car reliably available every day, or don't have a Jaguar Specialist who knows these cars in your area, don't buy one; and enjoy a really wonderful driving experience.