im thinking RS4, M3, 335i here, would keep it longer than a SEAT and shouldnt be too much more expensive as those cars are so perfect imo, that any mod you attempt could possibly ruin it. Or just go the Evo route, only issue being is that the cupra is the fastest car I can insure at the privelege of 2.5 grand so will probably get one and keep it for a year or two until I can afford to insure a grp 20 car.
Like I say, all the cars you have listed are in a different price category.
Also don't be under the illusion that you will not succomb to the desire to tune something more exotic like a 335i. Yes they start as 307bhp, but they tune quite easily to 370bhp or thereabouts. Then you could start to change components to further that again.....
Anyway, going by your sig, a Cupra at 240bhp is gonna be a pretty decent step up anyway which you could also keep standard for fear of spoiling it. However, for tunability with stealer back up, an Astra VXR with a stage 3 conversion may be a better bet for you. Just under 300bhp would be available for a few grand over the basic price of the car.
Also bear in mind there is the Vauxhall equivalent of the VAG TFSi engine just round the corner for the next Astra VXR, with a similar turbo and tunability to the likes of the Cupra. If the current Astra VXR is something to go by, no doubt tuning kits which are warranty approved will be available for this car at your local Vauxhall stealership
340bhp or so for just a couple of grand, complete with full warranty? Nice option to have. Just a shame its on a Poxhall.