Louie,
Is it possible that the titles to these cars may not be in your name? Here in Ohio, if a car is titled to you, you need your own insurance. Ohio is the cheapest auto insurance in the country, and premiums are based on age, # of years driving, and credit score, all of which I would hazard a guess you have very little of. In Ohio, a 17 year old cannot have a credit card in their name, and a parent would have to co-sign any revolving credit account.
If you did have your own auto insurance, I would venture a guess that your premium would be at least $3,000-$4,000 per year for a $50,000 - $100,000 Benz (of course you would want a Stated Value Policy with full coverage, and would have an appraisal), and have multiple vehicles. I know you work hard to earn money, but my kids make that with a whole summer job. Of course there's school during the year. Drat.
You did say you purchased the car from your mom, so Title would have transferred to your name requiring your own insurance, unless you co-titled the vehicle, so you only own half of the vehicle, and your mom could put the car on her policy. I could be wrong with my analysis, but this really isn't making sense to me.
So my question is, do you even own this 280SL?