there are many distros and normally you may choose one that you will feel comfortable with. I have used RedHat for many years and there are peoplr that hate and there others that love it. I think for backend and stability servers I may have to go with Slackware stable.
Ras= ( I would rather use a VPN try using Freeswan it has ipsec and everthing)
Firewall=iptables but there are lots of other like smoothwall,etc
DHCP= linux has it on system buy default may have to config files only