You've made some interesting choices here. Lots of people here are questioning using a gateway, but I think this is a great choice for what you are doing, and required to get decent battery life. However, I think using ZigBee will end up pricing you our of most markets in the long run. Especially if you elected to go with the very pricing Xbee modules from Digi. These are developer friendly, but way to expensive. If you are serious about building this as a product, I would investigate alternative RF solutions, there a lot of good ones out there.

Check out the 6lowpan standard, which is specifically designed for this kind of communication. Dust Networks (http://www.linear.com/products/smartmesh_ip) has a good implementation. The other alternative would be to go with some of the proprietary RF standards, I know this sounds prohibitive at first but will end up saving you money on your BoM in the long run. Check out Atmel's LightWeight Mesh (http://www.atmel.com/tools/lightweight_mesh.aspx) which is based on 802.15.4, the same as Zigbee and BT, so if you want you can even run ZigBee on the same hardware. Linx also makes a great proprietary RF module (https://www.linxtechnologies.com/en/home) which is extremely cheap even at low quantities.

If you are using Xbee (which it seems like you are) then you are likely not using a SoC, so designing in a different radio may still be relatively easy.

PM me if you have any questions, I've been working on this IoT stuff for a while now. Congrats on the crowd funding campaign and best of luck.