I lived in Koh Chang 4 years ago. There is a hotel called KB Resort (see map). the owners have several houses for rent near that area but without sea views. The price was not high, the houses are old but of good quality. Independent, quiet with garden. A vehicle is essential to get around.
Also, for long term you can Google DEE ONE PROPERTY, They are French and have a good rental offer throughout the Island.
Good luck!