Apt. in Sosua
There are all sorts of possibilities and things can change from here to September.
The ideal way to get a good deal on an apartment and get something that really suits you, is to come down, spend a few nights in a hotel, find what you want, end of story.
Shopping on the internet is difficult, while seeing is believing.
September is a good time for finding apartments, as it is low season and you will be able to make your best deal in person.