Internet through a NAS and Time Capsule
The NAS won't have any routing ability so it's pretty sure that it won't be directly connected to the modem.
Your schema is what I recommend. I have exactly the same architecture with a modem, an Airport Express and a Synology DS215j.

Modem ---------------- TimeCapsule (as router) ------------------ NAS
       WAN Interface               |              LAN Interface
                                   | LAN / Wi-Fi
                             Other devices


You may need to configure your TimeCapsule to make some Port Forwarding if you want to access to your NAS from outside of your network (I highly discourage that because of network security issues).

That all for me.