Most networking devices support SNMP, including many WLAN accesspoints. However, your question is not possible to answer in the format of StackOverflow. SNMP is a "decentralized" protocol, where each device vendor can decide exactly which functionality should be implemented by the device. The vendor publishes a MIB (Management Information Base) module, describing what variables can be monitored. Each device can implement any number of MIB modules, including ones published as Internet standards.
Certainly, SNMP will be a central part in any enterprise-level Network Management solution, but for such a large network, what you need is probably a good technical consultant (or better yet, hire someone with network management experience). Or, reserve a few weeks and prepare to do some extensive learning. ;-) The RFCs that define SNMP are a good start, for example "Introduction to community-based SNMP": http://www.ietf.org/rfc/rfc1901.txt