Check what to do wrt. NetworkManager's internal DHCP client vs. isc-dhcp-client
network-manager (1.2.4-2) unstable; urgency=medium * Demote isc-dhcp-client from Depends to Recommends. NetworkManager will automatically fall back to the internal dhcp client if dhclient is not available. (Closes: #826680)
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=826680 for details.
It seems to me that if the internal dhcp client has the features we want (e.g. wrt. not sending the hostname on the network), then we should go for it: isc-dhcp-client is lots of old code, and has a history of security issues.
At the very least, we should make sure that whatever happens, without us changing our Git tree, works and is safe.