Is there anything special to look out for when inspecting a 1970 Dart Swinger?

I don't understand why the op is considering spending $12k or more on an unknown car with potentially big problems instead of spending the money on the car you already have and know well?

If it was me I would buy something that would get me through for the short term and work on the El Camino whenever time/money allows.
Just my .02.