Well
HHF, I used to live on Koh Tao but that was a while ago, it has probably changed a lot since I was there. The island isn't that big with basically one main road that goes from one end to the other.
As far as diving operations are concerned, there are plenty to choose from and everyone will give you there own answer, some of the better equiped are the likes of Bans, Planet Scuba, Big Blue, New Way, presuming they are all still there but should be able to
Google them all.
Lots of accomodation available to fit most tastes I think from what I hear now. When I lived there there was no ATM's, no mobile telephones, few cars/bikes only really. Now I hear it has everything. Lots of good bars and restaurants all over the place, no problem on that front.
Many of the diving schools have their own accommodation aswell. Sarree beach is a lovely place to stay and that's where many of the dive schools locate their main operations with the exception of Bans diving school which is on the other side of the island.
Great diving around there and you could even see some whale sharks, if you're very lucky, but they do go to the islands.
If
Crazy88 sees this then he will have some good input for you aswell.
EDIT: Forget to mention that many of the dive schools have an office around where the ferry drops you off to the island. Some of the big hotels round there.