Oh, I think you will love it! It will be a massive difference.
I actually drive my parents Nissan Quest back in the day and bought a 2002 WRX as my first car when I was 18. It was a lot of fun.....a STI though....that will be just incredible for you haha!
By the way, since your from Germany, if you haven't watched this review on the STI yet form Autogefuhl, I highly suggest the watch!
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Also, as @
Boilermaker128 mentioned above. I also suggest you just keep the car stock and find a unmodified STI. In stock form they are reliable and honestly it doesn't cost a lot of money to maintain it. It will be horrible on gas though compared to many new cars today ^___~
I don't agree about only keeping the car for 5 years and move on to your next more expensive dream car......STI is my dream car personally and I don't see the need to spend more money on more expensive cars. Be smart and invest the money into other things, not just cars.