Talk to the people in the following thread about using your nest devices with smartthings. You might be able to use either IFTTT or the custom code in that thread to use nest cameras as your Wi-Fi motion sensors.
Another good option that works for many people as mentioned in the wiki article is to add a Hue bridge in the outbuilding, Have hue motion sensors Turn on hue lights in the outbuilding, and then have those lights coming on trigger smartthings events.
That’s a combination of LAN and zigbee, But the zigbee is communicating to the hue bridge as its own mini network. The smartthings integration is via the communication to the hue bridge. You will probably need to put a Wi-Fi access point in the outbuilding that has an ethernet port and plug the Hue bridge into that port. That should get everything connected to your LAN.