The cambelt should have been changed twice by now, so make sure it has or be prepared to have it done.
JTS engines are known to develop injector issues at high mileage, so check it runs smoothly and there's no hint of a misfire.
You'll get about 30mpg from the JTS and around 50mpg from the JTD. I've owned both engines and the JTS was all round more fun with the Diesel better suited to motorways. However debate rages on here about the merits of either!
My GT JTS was pretty reliable. It shares the same crap front suspension as the 147 and 156 which means the upper wishbones get noisy and all you can do is replace them unless you're happy to turn up the stereo and ignore the noise.
I've just sold my GT JTS and I'm going to miss it. Just check that the one you're looking at has been looked after and it'll serve you well.
"Nothing that has any meaning is easy. 'Easy' doesn't enter into grown-up life"