They will charge you 1200 baht for your Cambodian visa at that crossing. If you have a passport photo you might be able to get it for 1100 baht. It's worked for me before. If you have a few days before you go get an e-visa online here:
Cambodia Visa, Apply Cambodia Visa Online, Cambodia e-Visa - Official Site for $25US.
Your single entry visa will be void when you leave Thailand and you will only get 15 days when you re-enter. If you go to an immigration office before you go, I think you can get a re-entry permit that will let you continue on your single entry tourist visa. Another member will know the answer to that.
On Koh Kood you will be able to book tickets for travel to Phnom Penh or Siem Reap at most guesthouses.