Good to see I'm not the only one who travelled quite some distance to get my Z, 4 hours each way! Was good to have a long first drive in it though
I have the 156 and at the moment I'm more than happy with the performance but saying that, I did test drive a 200 and there is a noticeable difference, so would definitely go for that in your case as you're coming from an ST. Would definitely go for the GT spec, there's not too much price difference in terms of used RCZs and it's well worth it for the bigger wheels and heated leather interior (tyres aren't cheap on 19s though!)
One way to see if the vision pack is fitted is to go on the menu button on the CD player, and there's a setting on there for directional headlights - I'm pretty sure these only come with the vision pack, although I'm happy to be corrected if not!
In answer to your question about the leather, mine is a 10 plate with 41k on the clock and the leather is in very similar condition to my parents' 1 year old Range Rover. This is probably partly due to me using products on it regularly to keep it in good condition, but it's a fairly good indicator of quality. I saw a thread on here where someone mentioned 'bum craters' starting to appear in the drivers seat, and they are beginning to appear a little bit in mine, but not really more than I would expect, and it's the same with any leather seats I think - my Astra Convertible on 70k was much much worse.
I also looked at TTs and am glad I didn't go for one. The Z is much more individual, plus an equivalent TT is often another 5 grand! They're still lovely cars, but not worth it for me personally. That's not to say I'd rule it out completely - the new one especially is a superb looking car.
Good luck with the hunt, I don't think you'll regret going for a Z - I definitely don't! :beer: