1) SEAT charge £169 in the 1st year and £289 in the 2nd. The diff and gearbox also need servicing but not until years 3 & 4 respectively.
Regarding tyres, how long is a piece of string? This will heavily depend on the tyres fitted and your driving style. Most cars come with Continental Sport Contact 5, which are rubbish. Many people only get 5-8,000 miles out of them. For a Cupra, the best tyres by far are Michelin Pilot Sport 4S. These often last 15,000 miles. Typical prices have been £160 for the Continentals and £185 for the Michelins but Black Circles are currently selling Michelins for the same price as the Continentals, which would be a no-brainer. Does your contract allow you to choose the tyres?
2) KESSY only comes with the Cupra Lux, not the Cupra, which has an old-fashioned key socket on the steering column (i.e. the start button only comes with KESSY).
The reversing camera only comes on Leon Excellence. Cupras only have parking sensors.
3) MY 2018-on the LED lights are pretty good but not as good as bi-Xenons. You'd need to test them to see how much difference you notice.
4) It is recommended that you run it on 98+ octane all the time. 95 octane can be used in an emergency but the ECU will retard the ignition, costing you power and economy. Tesco are currently selling Momentum 99 cheaper than diesel though (at least in my area).
5) The 7-speed DSG was introduced after production restarted a few weeks ago after a break for WLTP testing. My understanding is that an cars with the EZ designation are 7-speed but I couldn't guarantee that. 7th gear is any overdrive so will be quieter on the motorway, yes.