Hi from another newbie. To answer some of your questions:
My classic Impreza Turbo was around £400 a year to insure, strangely my new STI is less than £240 a year to insure with 12 years NCB and I'm 38 years old. How insurance is calculated is a mystery to me.
I wouldn't bother trying to mod a non turbo car, it's not as simple as just bolting on a turbo charger.
Not sure about noise levels. My old Impreza was loud, used to wake the neighbours when I worked early morning shifts and noise never got mentioned on MOT's. Just got to watch emissions, had to make sure my car was hot when it had emissions test.
As far as I know you can own as many cars as you can afford to tax and insure.
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