It's not hard to find a unsecured auto loan, especially online. With a job and a bank account you can usually come out on top. Though it's going to take more time if it's a Mercedes-Benz you're after. It would be wise to start out small.
Don't fall for the tricks that are out there within dealerships. Though some of them say that they will finance you. Be weary. Some dealers when looking at your credit score will try and take advantage of the fact the you are desperate and that your credit is obviously in the dumps. Some will try and talk you into things that you don't need and will state that "because of your credit your options are limited". This may be true to some extent, but don't depend on them to deliver the big picture in it's entirety. Some will state that the lender that they work with will only approve you if you take the most expensive warranty. They will also make your less than perfect credit out to be worse then it actually is. Some will play on your bad credit as if it were a miserable extension of your self worth. You just want to buy a car!
Instead of graveling at the feet of these crooks try and situate your loan first. Then you can go in the dealership and say you have less to work with then you do. Make the dealer think you have less and you can probably avoid paying as much, as most dealers are as eager to sell as you are to buy. Having a trade in is always good but not having one shouldn't stifle you under the right circumstances.
Having a firm game plan going in can ensure that you win. Maybe not the vehicle of your dreams but something "new" to you and reliable. Case models, prices, dealerships, and always cover all angles when formulating your game plan.