There are several ways that I have in mind - but "the" best solution for you is pending on your personal requirements:
1) Work and store your files on the NAS only
3) Use a Backup SW on your Mac to sync your Mac Folders with your NAS
I personally use this approach, because the NAS could be used as a "single point of data store" for several PC's/Mac's.
The today NAS devices like QNAP delivers a lot of services and the stored data could easily backup'ed to an additional external USB HD, cloud storage service or second NAS in case of a hardware issue. Also the NAS has low power consumption.
Enable the TimeMachine service on your NAS and on your Mac and exclude all folders except the two you want to backup.
I tried "GoodSync" as a SW solution, if you want to sync some local folders to an external drive (for example, your local folders on your Mac to a NAS).
The Software is for free and you could download and test it from http://www.goodync.com/mac/download