FDS is a LDAP directory. It was the Netscape directory that Red Hat purchased from AOL and then opened the source. The Red Hat directory is the commercial version of FDS. . See
http://directory.fedora.redhat.com for more info. FDS and Red Hat directory is well documented. It is a proven product and is easy to install (compared to openLDAP).
Yes, samba can use a LDAP backend. Samba schema must be imported into FDS and samba has to be configured to access FDS. My suggestion is to setup a test server and give it a try. It won't be easy like SME.
I have not used SME since version 6.01, so I can't comment on samba or backup situations with SME 7. Using FDS with centos or fedora is effective but you will need linux experience to make it work.
Good luck.
ryan