'sup. I am 20 with 3 years no claims and insurance for me on the IS200 SE which I bought yesterday is £1400 (with Direct Line), so *** knows what you're paying!
Regarding SE vs. Sport, the only real difference is that the sport has a differential on it. I would suggest (as I've just bought one) that you save yourself the extra cash which the sport costs and get an SE.
With regards to mods I suggest that you live with the car for a bit and see how it suits you before rushing into anything. My experience is (being young also) that most insurance companies won't touch a young driver who has done any mods.
Finally, out of interest - what are you getting quoted on insurance and who with?