A full time Instructor course should take about 8 weeks. Around 80 - 90 hours of PMI and briefings should include some demonstrated briefings as well as some you prepared. After all that's what the PMI is for. You should make firm bookings for the whole course and if bookings are not kept, demand to know why. As many have said, YOU are the customer. But you are also a potential employee under training.
As others have said, if possible, do the training with the CFI and hopefully s/he is the ATO as well. You would not want to fail the test just because someone else skins a cat differently.
Don't be afraid to move. The school will have to transfer the records to the new school. The new school will make sure you are up to standard but you will not have to do it all again. It does not matter whether you are paying for a GFPT, PPL, CPL or Instructor Rating; you need to be comfortable with the aerodrome, the aeroplane, the operator and the instructor. If you are unsure of any one of these, you progress will be impeded.
And one last word. When you have your rating, don't visit the same sins upon you students. Best of luck.