Hi, Brian!
the only one i'm concerned about are shared access data bases but I think I can make these work too.
First: You will probably NOT be able to get these important and ugly "MaleWare-Access-Based-Office-Software" to work on E-smith/Samba. At least I tried with a very common german lawyer-application "RA-Micro" and i tuned all the samba stuff and it never worked. If you use that kind of stuff, then you should be very careful not to disappoint the management.
But the rest is simple:
Just set up a decent sme-box. Do not configure as "primary-domain-controller", for this will mix up the whole stuff. Create ibays and users for your needs. You have to do this manually, because e-smith does not support samba 3.x by now.
Copy the non-access-based-stuff to the new ibays and map them as local drives in the NT-Login-Script. Just change the NT-Login Script from MAP G:\ \\NT\DRIVE to MAP G:\ \\E-SMITH\IBAY (not shure, if this map command is correct from here).
If you do this by night, the users will not even realize the new server.
Then: Set up an Ibay for the access-stuff. Copy and test it. Perhaps it will work - perhaps not.
If it works, again: Copy and change map-command.
Last Step would be the most complicated: For migrating the profiles and home-directories you will need better guidance than mine.
But this will do for the first test.