1. Cambelt/waterpump: - Both should be changed every 60k or 4 years whichever is sooner. Most cambelt failures are due to seized water pumps so changing the belts etc at the correct interval will avoid this.
2. Clutch: - Most GT/147 owners say that the diesel clutches are heavy so don't be alarmed, however, if the clutch bites high it might be nearing the end. Clutch replacements will set you back around £500 and you can double that if it needs a flywheel. Some haggling is in order if this is the case, if not, excellent.
3. Bodywork: - The panels are quite thin and dent easily so check them for damage, also, check the sills as it's common to find damage from incorrect jacking mostly behind the front wheels. Rust shouldn't be an issue but have a good long look around the car for corrosion, especially the floorpans, don't be afraid to give them a good prod from underneath.
4. Engine: - The JTDm's are strong and if cared for properly will be reliable in most cases. You may experience the usual EGR issues, whipping it off for a clean every so often will help prevent it sticking open. Low-down power/stalling is a common sympton of a stuck EGR but it's a simple fix. The lower turbo hose is known to wear against the gearbox causing it to blow, upgrading to the silicone hoses is recommended when the time comes as these expand much less than the OEM part. Viewing the lower hose is difficult as it's hidden by the undertray but excess black smoke when accelerating is common if the hose is split. Oil consumption isn't an issue but if it's low I'd walk away.
5. Electronics: - The electronics on the 156/147/GT are generally reliable, the most common issue seems to be with the windows on the 'frameless' doors of the GT. Evil, horrible demons are known to live in the GT's doors causing much annoyance with window operation. A simple window reset usually solves it (touches wood).
6. Suspension: - Check for any knocking, the front upper wishbones are known to squeak when worn and cause uneven tyre wear. Although the GT likes to eat the inside edge of the front tyres, anything excessive could be a tracking issue or worn suspension components.
There will be things I've missed but it's a good place to start, this guide goes through more common issues: - Alfa Romeo GT Buyer's Guide
147 Sport Q2: Autolusso Remap + Swirl Flap & EGR Delete, Mocal Oil Cooler Kit, JTD Performance FMIC Kit + High Flow Elbow, Wizard Downpipe, PF Pads + 5.1 Fluid, Autolusso Braided Brake Lines, MTech Short-Shifter, Autolusso Silicone Turbo-to-MAF Hose, Refurbed Q2's + Some to add later.