Well, considering you're 17, you won't have a full years NCB and insurance is going to my seriously high anyway if you're on your own policy (which you should be, as it's your car).
Basically, you're in the same boat as me, and for the moment I reckon you should stay away from lowering/changing kits, as this will make insurance much dearer! Stick to the aesthetics for the time being, such as a splitter/alloys, and perhaps check with your insurance company how much a Seat standard bumper would be to add.
I'm already being battered with a £2500 premium, with my no claims coming up in October, so am still going to wait for a bit until I bother with any serious changes.
Search this forum, as there are a few people who have successfully changed their front and rear bumpers to Facelift 06-08 versions.
What alloys have you got your eye on? A Cupra set, or aftermarket ones?