Hey,
I did a little research on what you need and it's a web developer that preferably has experience with Squarespace's developer API, so this would not be related to either Mac or C Programming. :).
I do not have previous experience with Squarespace, however, i took a glance at their developer documentation and i can resonate well with it. I can't provide a right budget or a specific deadline date because i don't know what are your all demands. But I'm open to dialog :)
Cheers