Good man, the GT is a great looking car. I have a 147 with the same diesel engine and to be honest it's more economical than I thought it would be. It's rapid enough and you get plenty of torque. I get around 42mpg on a mix of country/city driving. You're absolutely spot on with the belts and water pump. The timing belt and water pump should be changed every 60k or 4 years. Apparently it's more likely for the water pump to seize causing the belt to fail rather than the belt itself failing. For me I'd like to see a full-service history with regular oil changes, if the oil itself or oil level show signs of neglect you may wish to seek a better example. The JTDm is a strong engine and with regular maintenance it will last, this is important if you're covering 20k a year.
Have a good look at all 4 tyres, it's quite common to see some inner edge wear on the front tyres due to the cars setup. However, excessive wear can indicate worn upper wishbones and/or the anti-roll bar. Budget about £100 per side for wishbone replacements and around £100 for a new ARB. Rear suspension arm wear can cause uneven tyre wear and excess noise from the rear wheels. There are four rear suspension arms, each pair is a different length and they cost around £50 each for the longer and £35 each for the shorter plus labour should this repair be required. With 150k on the clock I'd also want to check the front lower wishbones, see if the owner will let you have it checked over because if it's ready for a suspension overhaul it will set you back a pretty sum.
Check the clutch for any slippage as the cost for a replacement in the diesel can easily reach £900, mainly due to the dual-mass flywheel. Check that it pulls strongly in all gears with no hesitation. As with all Alfa's ensure that you check everything electrical, the 147/GT uses canbus electrics which can be difficult to troubleshoot. In particular GT's are renown for their front windows to become possessed by a force of evil, a simple fix so I understand but annoying nonetheless.
There is a great buyers guide on the Alfaworkshop website which should give you all the information you need, I've just outlined the most common areas.
Alfa Romeo GT Buyer's Guide
Best of luck!
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.