as you are planning to go ahead with Cisco equipments for network upgradation but it always better to planning per future but not per your current requirement, as per your provided details you have about 400 nodes in network so for CORE managiability...

Before going to buy equipments it is very neccessary to know what kind of servies you are going to use in your network, whether it is only data or Data and Voice both, accordingly you may go ahead with equipments planning.

I guess you may facing some P2P link issue with ISP, loopback on CKT always going to use by your ISP, what they do, they set soft loop or hard loop at your last mile ISP provided device after that you will get "loopback set" on sh interface command w...