If I were to consider fiber-cement siding for a cottage style beach house I would look at Hardie Artisan siding. Its a bit more than 40% thicker (5/8 vs 7/16) than regular Hardie siding so it can be mitered at the corners. The Lap (4, 6 & 8" exposure) and Beaded Lap (7" exposure) versions have T&G end joints so the seams should not be an issue. The other versions (7" V-groove, 9" Shiplap, 9" Square Channel, 9" Bevel Channel) all have horizontal "lock joints" so they're easy to install and the end joints should not be an issue.
All of these versions can be mitered at the building corners. The window trim should be 1" (5/4) cellular PVC or composite with a similar sill added.
The Artisan sidings are primed and will need to be field painted.
As for cost I suggest not asking if something is more expensive than another thing but competitively bid both designs and see what happens.
For more specific advice you should post the design of your house.
Q