there are a number of good IDC centers and Course Directors on Koh Tao. But you have a more immediate problem. If you are wishing to become a PADI OWSI, then there is a 6 month wait from the time you certify as an Open Water Diver to the time you are allowed to participate in the IE Instructor Examination.
i would suggest that you take an OWD class and get certified, then get some diving experience, and look into obtaining a complete kit of gear (pro quality) before you settle on where you'd like to do your professional level programs.
where you choose to do your professional level programs should be driven by some factors like where you'd like to work.
Cheers,
Ken