Welcome!
I get 26mpg average out of mine, all driving taken into account, using Shell V Power. 95RON takes the consumption down a bit.
Insurance, as has been said, is a lottery. Mine at 25 is about £600, 3 points, 2 years NCB.
Mod wise, plenty of visual bits available, from tacky stick on, to top notch Jap tuner products. Speed wise, it's not slow in the first place and then noticeable gains will need £'000s spent. Subtle changes in exhaust / induction / plenum will add slight changes.
For any faults, take a look in the warranty section here. How old a car would you be buying?