You just have to look that the settings for encryption (IKE, IPsec), the lifetimes and the DH group of them is the same on both devices. No big deal to make a Site-2-Site VPN with a ZxXEL device.
You only have to know one thing with ZxXEL devices. They often only start the VPN the first time when there is data. So, always ping a device on the other side when configuring it. I spent maybe an hour the fist time i tried it because i never pinged (or accessed) a device on the other side and always thought there is something wrong because it always showed the VPN as down.
