rent a jackhammer and plow through all that concrete!

Depending on the house, that may not be the best way to do it. Speaking from experience, here...

When I was involved in a similar project, we, uh... discovered... the main sewer pipe going under the concrete. It was a ceramic pipe. We had to replace a pretty good chunk of it after we shattered it with the hammer. In the end, we built a 4" platform for the toilet to sit on.