Wow. I don't see how anyone will be able to say much over the Net; you need boots on the ground for this one, IMO. Anything goes for foundations, or anything else, prior to codes and enforcement. Is it feasible to jack the houses up, install a proper foundation, and go in peace? It may even be advisable to install conditioned crawl spaces. What is the land worth, btw? Maybe you can look at it as buying land and putting up simple, new, and lasting houses. ??? Good luck, either way.