You have to consider your own situation. Is $550 a lot to you? Do you have other disposable funds right now to get another car?
You could go out and pick something up for $500-600...but that can be a gamble. I just did this about a month ago, and the car I got has been pretty good, but not without a couple (minor) problems. The last car I bought ($700) before this one was an absolute mess, and the car I got before that ($200) lasted me 4 years with almost no trouble. YMMV of course.
Or, you can fix the car you have with that $550 - that way, you know the car's history, you know about the car's maintenance, you know what has been serviced recently and what hasn't, you know about other things that may need attention on this car, etc. It's a lot less risky.
I would probably fix it, if the body was ok and everything else was in decent shape.