1. Are your remote clients using Secure Domain Logon (SDL)?
No - We decided against this as a security risk, we didn't want the VPN up before the user was logged in. Instead we use User Certs stored in their personal store so in order to connect to the network they must first authenticated.
2. Do you serve them IPs from AD DHCP or the Office mode range?
Originally we used DHCP from AD, however we do not have AD in the Datacenter that the Remote Access firewalls are located in and this meant traversing other firewalls and a VPN tunnel in order to get IPs. Given the dependency on this we moved to the firewalls providing IPs locally which meant that we had to add logic via a post connect script to update DNS (only secure DNS changes are allowed in our AD) and for updating GPOs.
3. Do you allow Reverse Lookup Zone in AD DNS to use Nonsecure and Secure updates?
Secure updates as above, this is handled via a script run post connection from the end users device.