Hi,
I have not used IOS DHCP but would like to comment on your question. Doing some reading on it have you consider using DHCP database agent.
you could setup on your current syslog system a FTP server for storing dhcp leases information from the router.
Configuring IOS DHCP Database agent, this could give you more granularity information in ip lease time etc..
http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a008008743b.html
Configuring the router to query DHCP database server ( FTP etc.. )
ip dhcp database url [timeout seconds | write-delay seconds] ( keeps track of IP/MAC leases info etc.. )
http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a008008743b.html#36847
as far as the level for this type of query I would say it could be "notification level 6",
notification levels and facility levels
http://www.cisco.com/en/US/docs/storage/san_routers/sn5400series/rel_3_4/command/reference/lcli.html#wp2414106
HTH
Jorge
Jorge Rodriguez