You're not the first to mention that, but I still find it surprising. You'd think Google would hit on JaguarForums.com or some of the other dedicated forums first, but I guess their search algorithms kinda suck at that.
Honestly, if you're buying it unseen and unevaluated by a mechanic experienced with the car model, have it shipped. It's cheaper than a major mechanical breakdown thousands of miles from home and you can do your initial working up close to home instead of doing
Hot Rod's Roadkill for yourself and hoping.
I would say that while it would seem like a low mileage example is the way to go, with these classic Jags, a somewhat higher mileage example is often the smarter buy. These cars really, really, really hate sitting and a 1992 that's got only 28K... well, it's likely to be a bit unhappy if made into a regular driver. You can expect to need to replace everything external that's made of rubber - weatherstripping, engine seals, underbody seals, door seals, etc., etc. You can also expect to have to replace the convertible top material (due to age if nothing else, even if it's lived inside its entire life) and possibly replace the electro-hydraulics that raise and lower the top. You may get lucky and not have to do that, but figure on having to do it anyway. Figure about $10-12K or so, though not all at once in a lump sum. Substantially less if you plan to spin your own wrenches and shop carefully; also, once fixed, most things on the car will *stay* fixed for a good long while.
You should also demand and require a service history on a car this low mileage, because since it was so rarely driven the owner may not have done any maintenance. Maintenance is required on a time elapsed basis as well as mileage - and sadly most people don't understand this. Brake fluid should have been changed every 2-3 years, coolant every 3 at most, oil every 6 months or at worst annually. If you don't have all the service records? Run - low miles or not, you're looking at your expenses going from significant money to OMGHUGE because you're looking at potential total system overhauls. V12 overhauls are, uh, not cheap. Tack on a 4L80E rebuild, which while not outrageous is still going to be a low four figure job to get done right, an 'everything in the rear subframe rebuild' at about $3K (all prices including labor), etc., etc., and you're looking at a potentially bottomless money pit.
If you want someone to look over the car, I can give you some references to skilled and experienced classic Jaguar mechanics (sadly fewer every year it seems) here in the Dallas area that would be happy to help you. Expect to pay them for at least an hour of their time to properly go over the car once it is presented to them - with a convertible I'd say perhaps even an hour and a half.
All that said? While I'm not the XJS guy, they're still a classic Jag. They're still like literally nothing else on the planet and if you get a good one, you'll never regret buying it.