Werd up.
Insurance and modding is a messy subject. You need to do lots of ringing around and get online quotes to get the best deal for you. You can pretty much write any 'performance' mods off right away
No engine transplants, no turbos, superchargers, throttle bodies anything that hikes the power up is a no go. Air filters and aftermarket (stainless) exhausts are frowned upon and will again hike up your premium. However, it might be a manageable amount. We might be talking +£100 to your overall premium for these items.
Cosmetic mods like sideskirts, arch extentions, bumpers, spoilers, alloys etc, will also push your premium up, but more companies will let you do this. Average Joe will consider putting alloys on their car, so it's quite normal. Full PSG Widearch kit however, is a little extreme so expect big bucks to be asked for.
In short. You're fucked.
But if it's any consolation we've all been there. If I were in your position (and I have been) I'd spend a little more on the insurance, but keep my car standrd for a year, hopefully earn your 1 years no claims and then in the second year look into some mods.
The 16v Si is a great first car, some might argue a little too quick for someone inexperienced but I was fine
A decent standard Si can be had to a reasonable sum. It's modern enough to not suffer majorly with problems and has a few nice creature comforts. (Especially things like the interior!) In my slightly older age, I've come to realise it's better to buy a car you're pretty happy with as standard rather than buy a 1.1 popular plus that you know you're going to want to mod. 
