If the water "doesn't go down", there's a clog very close to the toilet, perhaps in the toilet itself. Pull the toilet and have a look. If that's OK, I'd hire a plumber with a camera to have a peek down the toilet(s) all the way to the sewer (or septic?) (which?). Does water "go down" other fixtures? Do you have a diagram of the plumbing systems, or a basement where you can see it all exposed?
Foreclosed houses have lots of risks. If the foreclosee was really upset, he could've flushed a bunch of diapers and a bag of quick-setting cement down the toilets. "All the pipes are frozen" sure isn't a good sign, especially if they were truly winterized. Have you tried turning on the water and seeing where all the leaks were from all the broken pipes? If so, you might have recourse against whoever winterized the house. But if in fact it's as bad as it looks, re-plumbing the entire house, supply- and waste- side, with all the demolition and reconstruction required, could well run into very big bucks.