I'm a Web Admin... AKA I do PHP work.You make $33 dollars an hour? As an 18 year old highschool kid? My first summer internship for an multi-million dollar electrical engineering consulting firm I only made $30 an hour (plus living expenses) and I had three years of school in one of the harder undergrad majors out there under my belt.
Bullshit.
Trade your car in for a nice used one and get a nice used bike. But keep both within your budget, it can be done. I did it before and was happy with my decision. With 15k you can buy a nice bike for 4-5k or less. Have 10-11 or more to buy a used car plus your trade in. Then you have the both of best worlds. Check autotrader.com, even if you have to drive a few hundred miles to save a few grand, it's worth it. With aftermarket warrantys these days you can take it to a lot of places in your area to get it fixed without a problem. Make sure to check your insurance on both before you buy them, some vehicles and bikes can get costly.
Honestly, you're 18 and don't really need any speed. I would get a nice car thats moderately fast that you won't feel the need to mod. That takes the Nissan out. You also don't want anything that is too expensive to maintain. That takes the BMW out. Do you necessarily need a luxury car at 18? Or is this so that you'll be accepted by your friends and have to drive them everywhere? The IS sounds like a good deal as it doesn't require as much maintenance. However, I'm hoping you'll be doing all of your own work on this thing, otherwise it will get very expensive. Don't look at the initial price of the car, rather how much it will cost you to keep.
And getting a bike is very impractical.