They really are great cars, I've had 3 of em. Things to look for are:-
1. Obviously Service history - regular oil changes
2. When cold the older ones, specially getting on in miles start to rattle from the timing chain. Usually a sign of the tensioner wearing out. It does disappear when the engine warms up.
3. PAS seals tend to go - look for any leaks from the rams
4. Check discs & pads - neglected ones (discs warp) which leads to uneven braking. & if the pads wear right down, it pops the seals in the caliper for the piston & seizes it.
5. Dash warning lights - Again older ones throw up warnings for fun when really there's nothing wrong (light not working springs to mind).
6. Check suspension & springs - These are bloomin heavy cars & if no work has ever been done on the suspension check it over.
7. Spare key(s) See if there is one, if not at least get the key code. If not Jaguar charge ££££££££.
It says on the add its a 5 spd manual, but lookin at the photos of the interior it looks like an auto???
Things I've mentioned to check are only guides what to look for & things either I or some of my mates have experienced. I love em to bits & the straight 6 is absolutely bullet proof if maintained (unlike its sucessor, the early Jag V8). Only reason I got rid is cause I got married & couldn't afford to run it.
It does look like a great motor and for £1200!