Tips on Buying Cars: PLEASE HELP!, median score, disability income

Question
Dear Sir:
 I have a bit of a sticky situation and really need some advice.  The car I have now has a lot of things wrong with it, including that it needs a new clutch (so it won't be in running condition for long).  I hate the car and don't want to put any more money into it.  I am trying to sell it for $1,500, which is reasonable.
 The bigger problem I have is I need to buy another car!  I already know what I want, and it will cost about $5000-6500.   I don't even have $1000, so I would need to use whatever I get for my car as a down payment, and then finance about $5000.  I have only bought cars outright before so I don't know anything about financing a car.  Let me tell you the variables of my situation and I hope you can help me out!
1) I am unemployed and received disability income of $600 per month.
2) My fiancé will be co-signing everything, and she makes about $55k a year.
3) My credit is fair (median score is like 650) but her credit is poor (like 595).

Obviously my fiancé will HAVE TO CO-SIGN, but that's not a problem.  The problem is neither of us have good credit, and we're not looking to buy a new expensive car.  I don't even know where to begin!  

Please tell me what I need to do (in order to go to a dealership who has a car I want) and finance a car.  

Thanks you so very much,
Jeff

PS. Also, could you tell me (just your best GUESS) what kind of interest I would have to pay based on the income and credit situation?  I am only asking for a guess in this department.  

Answer
your in a bad situation....if you need a car right away...and you have no money the only thing i can say is use you car as a trade in and as the down payment....but the dealer will not give you what your car is worth and will raise the price of the car higher than its worth...so you lose money in both transactions....what i would do is find out how much your car is worth....try ...kbb.com....see what the dealer trade in value is and what the retail value is....then you will know how much you ask for it...then put it on craigslist.com its free and it works very well...make sure you have some pics...with the money i hope you make off the sale id buy a car....you can use craigslist.com to search for one....dont be afraid of a 1200 dollar car...there are good deals out there you just might have to look at a couple...id stick with a japanese car toyota honda or subaru with around 90 to 120 thousand miles...this will  save you years of payments at some outrageous percentage rate...build your credit back up save some money for a down payment in a year or two...if you dont have good credit and the car is over 3 years old you might pay as high as 13% interest...on a 6000 car with 1000 down and 13% interest you will pay around 120 a month for 5 years....and 170 a month for 3 years....but after 5 years the car is not worth anything and you might end up in the same position your in now....if you have to have this car....be smart and dont pay anywhere near what their asking...dont be afraid to say no and walk out.....they will call you back the next day with a better offer....see what trade in value is on the car you want....that is what the dealer payed for the car..now you see how much they mark up the price....any more questions ask....