Advice for my car shopping situation?

Ok so i have a 2001 nissan maxima needs a new cat converter 208 thousand miles on it. I want to get a new vehicle because of the high miles and it only gets 17 miles to the gallon. I know someone willing to pay me 2000 dollars. id fix the cat which is just under 1000 and get the other 1000 for part of a down. I also have 2500 cash. So thats 3500 for a down. My credit is in the 500's because i have no good credit and one bad credit which is a car repossession about 4 years ago. I went to big dealers because i was told they would try to work something out better than small dealerships. Basically they either said they cant approve me or i need atleast 5000 cash. Now i dont know if i didnt hassle them enough or what. I was told to try small dealers so im thinking of doing that tomorrow. Whats your take on this? Should i be able to get a 13000 vehicle with 3500 down? or what range should i be in etc. I just paid off my nissan a few months ago 355 monthly payments never late but it was in house so it wont be reported. Im stuck buying crappy high mileage cars the rest of my life!!! Im sick of getting a new used car every two years help advice!!!