You could try IP Messenger (IP Messenger
I have used it previously and it works fine on a local network and has the following features
* Serverless message communication
* Fast file/folder transfer
* Image embeded message, desktop (area specified) capture
* Message encrytion(RSA2048bit + AES256bit) and sign/verify (PKCS#1-v1_5)
As for the second requirement, it could be a bit hard to find an application that does it.