1) break up the soil
2) add bits of compost
3) plant legumes to add nitrogen
What else should I do, and what challenges should I expect?
i always see people say to take a sample to your state agiculture dept, they will test it to see what it needs. give them a call . or maybe a local collage with ag dept.
All of the steps you listed will help restore the soil. If the car parts leaked petroleum products into the soil you may have to remove and replace the soil. I would probably double dig the area and try to reintroduce more microbes into the soil.
The best way to fertilize your organic garden in the long run is to vermicompost. Vermicomposting is easier and has more nutrients than traditional compost. Just how the directions of the following website.