Here are some possible options.
1. Fedora Directory Server.
There are many tools out their to help you migrate data.
* export AD to ldif
* configure Domain / Ou structure in FDS
* modify ldif as needed
* import ldif
– Passwords can be a bit of a nightmare, but again there are many ways to kill this cat. Using AD NIS+ services could help with this. Its been a while for me it may be possible to just do some kind of kerberos sync.
* Split DNS into caching servers and Zone Masters
* caching servers only handle dns resolution. They are static servers that only need to be configured once. These would be your DNS resolvers. I use djbdns/dnscache for this part, it is an extremely simple to using caching resolver and it can handle a tremendous amount of load.
* auth DNS servers will be your zone masters. Use this to configure your domain names. Bind is usually used here (Webmin module for BIND is really easy to use)
If you want a really scalable high performance active distributed cluster fileserver on the cheap, you can’t beat GFS2/Samba3.3/CTDB/RR-DNS/FedoraDirectoryServer
* I find using something like ClearOS or Shorewall best for this, especially because of the admin interface. They are like purpose built security appliance distros.
* bacula (extremely powerful network backup) (my favorite)
* zmanda (commercial arm on amanda) – has agents for pretty much anything
* Check out http://www.zarafa.com/content/community
its a great replacement for exchange (for the most part), it sits directly on top of your existing servers (postfix, sendmail, courier, mailDB)
* Alternatively try Zimbra.
* Try alfresco (if you need all the features) or Knowledge tree if you just want a really good document management system.
As far as a plan goes.
– start with the dns and directories first. Get the windows and linux ones in sync, so that data migration will be eased. This part will probably take the longest.
– You could replace ISA quite quickly. If your using Shorewall or ClearOS for this task you should be able to replicate the configuration quite quickly.
– As for the rest, you decide.