I have no advice to offer on which car to go for, I always buy on condition, with service history a close second, gut feeling not far behind. I would recommend you do the same.
Get on your back and slide under the car, check the condition of the floor (should have underseal everywhere, some GTs/147s dont have any!), have a good look around the arches, check the subframes, suspension etc.
Pop the bonnet and check the condition of all the hoses. The upper and lower intake hoses are prone to rubbing through. I replaced my upper as soon as I bought the car as it was shagged, and literally 2 days ago, my lower burst in spectacular fashion - now sporting a Samco replacement.
Cambelt is obviously important, but a good specialist will only take around 400 notes from you to change (this includes water pump).
Others will disagree, but I don't think the upgraded dif makes much of a difference in day to day driving - yes if you're hooning it around then it'll be great, but the GT is primarily a cruiser. Turning it into anything else will be a long expensive, but satisfying, process (ask ALFADOIT!).
The Cloverleaf 18" alloys are a harsh ride, especially if the shocks and springs are tired but they really do look the mutts.
Colour wise, a black GT is a thing of beauty, but the metallic blue really shows off the lines.
Don't worry about whether its a 150/170 - same engine, different map. Sport button doesn't increase power, just throttle response.
Whatever you end up with, have Gus at Alfatune remap it for you - you won't regret it! Good luck!