My tuppence worth, in addition that I am also a crap driver, no crashes but I wouldn't know how to drift a car or drive it fast but happily took on the 350z as my first performance car after previously having a company car which was a 1.4TSI Scirocco. I found the zed quite forgiving if you drive within your limits and respect the car. In terms of your questions...
1. I got 23mpg average on my zed which was probably a 50/50 mix of about town driving and A roads - fuel prices at the moment are fluctuating, I wouldn't base it on todays rates which are pretty low and will climb again next year
2. Don't worry about the VED cost, it will be the lowest cost for running the car
3. Where are you located? Some very good trusted people on here service cars at good rates
4. Silver for some reason has been the most popular colour for cars for some time - red is an original colour, Chilli Red I think it is called
5. Budget looks fine for what you want, in fact cars in original spec can go for more than modified ones
6. Imports are absolutely fine, if its recently imported its probably going to be in very good condition, insurance is now pretty much on par plenty of specialist insurers will give great deals, my last two cars have been imports and both have been stunning cars
7. If you are buying now roadsters will more likely be at their lower end price as they are seasonal, if its just the roof slashing that you are worried about can you get a car cover to deter people? Personally I think they look awful with the roof up, but fantastic with it down...personal preference really