Pretty hard to say what you'll find under that slab, but there are some basics that will have to be addressed. First, the floor under the toilet has to be solid and level. If it is not, then you will have some more work to do. Assuming the old floor covering will be removed, you may find you will have to cut some of the floor and sub floor out and replace them. This should not be too difficult if the joists are in good condition. Of course the flange will have to be removed and be replaced later. The new flange should rest on top of the finished floor, be screwed to through the finished floor and into the sub floor, and be level. If the floor under the slab is already in good condition, then you just need to replace the flange. The flange is the last thing to do as it must rest on top of the finished floor be it vinyl, tile, or wood.