Based on 50K miles with mine:
Drive it over some rough ground and listen for any knocks or rattles from the front suspension.
Check the tyres for uneven wear front and back.
If the door handles don't go back quickly on their own it could be the hinges are starting to seize.
Tailgates can also rattle. Various reasons but relatively easy to cure on post 2006-ish models.
Look out for black clouds of smoke when flooring it in low gears. Jtd engine should run pretty clean for a diesel.
Check the radio finds stations. New aerial base is about £50 but is a pain to fit.
Finally, if the paint is red, look out for stone chips. If there are none it has been resprayed!
TBH not really a lot of significant issues with the GT
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2