The mk7 GTI engine has a timing chain rather than a cam belt. The chain is a non-serviceable item so in theory shouldn’t need changing, but it would be prudent to check it / get it checked for stretch on an early mk7 if the mileage is 80k plus.
In addition to the servicing items mentioned by @Guzzle and @Watts, if you did decide on a car with DSG transmission rather than a manual, the DSG should be serviced every 40k miles, brake fluid changes are every two years after the first fluid change, which on VW’s is at three years.
Cars with sunroofs - there have been some instances of the sunroof surround cracking in the area between the front corners of the tilting/sliding panel and the top of the A pillar. Replacement out of warranty is expensive. Check carpets for dampness as the rear door speaker seals on five door models are prone to fail, allowing water ingress into the cabin. Also check the spare wheel well for water, as the grommet on the bottom of the tailgate that holds the hatch release handle drain pipe in place can perish with age, allowing water to drain into the tailgate and then onto the boot floor and into spare wheel well.
If Apple CarPlay / Android Auto is important to you, then you would need to ensure any car you view has CarNet App Connect, or the ability for it to be retro-activated (cost to do this is likely to be around £250-£300). I’m not sure if early cars with MIB1 infotainment units had CarNet App Connect functionality - it was an extra cost option on later (2016) model year cars with MIB2 infotainment units. It can also be retro-activated on 2016 model year cars within App Connect compatible MIB2 infotainment units, but be aware that not all MIB2’s are App Connect compatible.