To my knowledge, teh difference between NetMRI and Network Insight is support for automation. Network Insight supports network discovery, adding that data to IPAM. NetMRI has the same engine and can update IPAM on a schedule, but it also adds network automation features which can push network changes, identify network issues, and store configuration changes a configured time period. If you only want discovery, go with Network Insight. You will need to work with your account team to size the infrastructure. You'll need more than one probe appliance for sure, sending to one or more consolidator appliances which will update IPAM.
