Something to keep in mind is that every lender goes by something known as "internal scoring" - in other words, they look at your history, and compute their own risk score for you. This score might be close to your FICO score, but it might be substantially higher or lower, depending on the factors that particular lender weighs more or less strongly.
There are several key factors that all lenders will take into account when deciding how risky you are, which will determine whether or not they will take the deal, and what sort of interest they will offer:
DTI/PTI
Lenders are really concerned about debt to income and payment to income ratios. As a rule of thumb, total monthly debt should be 50% (or less) of gross monthly income, and the total monthly car payments should be 20% (or less) of gross monthly income. The lower one's credit, however, the tighter those guidelines. With a score of 551, and not knowing why it is there, I would guess that most lenders that would potentially be interested in your deal would want to see something like 35-40% DTI and < 12% PTI.
LTV
Loan to value is another really big issue for lenders. Remember that car loans are an asset backed loan (in other words the value of the car collateralizes the loan). Unfortunately cars depreciate, and quickly, so in bad credit situations, lenders get pretty nervous. Also, keep in mind that most lenders will base their LTV calculations on the invoice price of the car. If you can close the deal with a total amount financed being $27,600 (a swipe at an 80% LTV on the 4.6 non tech) you will be much more attractive to a lender than if that number creeps higher.
Work History
Someone who has 2+ years at a current job for a third party employer looks a lot better to a lender than someone who is new on the job, or is self employed, or otherwise has trouble documenting their income.
Nature of Credit Blemishes
It is perverse, but some bad credit is better than other bad credit. For instance, if someone has a newly discharged bankruptcy, but no bad credit after it, they may actually look a lot better to a lender than someone who has never been bankrupt, but routinely pays bills late.
My advice would be to work with a local credit union first. You will get a much better idea of what your situation really looks like. Even if the CU can't work a finance package for you, a good dealership finance office might be able to find a lender, but be prepared for a high interest rate.
Also, keep in mind that once you actually get to the finance stage, most dealers will try to get you to buy a different car, so be prepared for a mental battle.
Good luck!