Are 700 THB fun dives gone? They should be there if you buy a package and not just one.
You'd normally do OW+AOW on KT.
For food, my usual routine is: get a Pad Thai with pot-brewed coffee (I'd prefer machine espresso, but it beats instant) in Coffee Boat on Sairee Beach (100 THB), lunch on the boat, dinner in Sairee Village (100-500 THB/person depending on how fancy I want to go).
Most people don't need aircon most of the year, though individual sensitivity varies. Diving without a wetsuit (which I strongly recommend there) cools down your core, you don't feel the heat as much, and it's not that hot.
No need to bother with hostels, it ain't Singapore. Just find someone who does monthly rental for bungalows etc and negotiate a sub-month price down from the monthly rate. E.g. 6,000 THB/mo can be turned into 4,000 THB/2 weeks. And with the kind of place this is, you want to stay at least 2 weeks.
Or much less (~2,500 THB or $80) if you accept the time expense and go by bus to-from BKK.