The best place to get tiling help is at www.johnbridge.com.
For tile to survive, you'll need to know the strength of the joists first. Then, you need sufficient subfloor strength because the floor can flex between the joists as well. Then, while it is possible to tile directly on two layers of properly installed plywood, it is almost impossible to get the right conditions unless you are dealing with new materials, so you need a properly installed decoupling layer - this could be cbu or something like Ditra from www.schluter.com.
How much you need to remove would depend on exactly what's there now, and its condition.