I think Eddie and the others nailed it. If it's cracked that bad in only 15 years, that's a clear sign that the soil wasn't prepped properly and any renovation is going to keep on sinking.
But I'm cheap. I think I would save the cost of a professional engineer, and just get bids from termite-repair contractors to level the floors. Then expect to repeat that work every 10 years.
It's up to you to do it right (teardown) or do it cheap and patch it every few years.