At the moment it's not fully clear how Lion licensing is actually going to work. It seems as though it's a download once, install many type deal, but again, not sure until release date.
But you need at least one Mac with Snow Leopard on it to download Lion (App store only. booo) If you were looking to use one of them as a server instead of a workstation then it might be worth waiting for Lion as there is no server OS X anymore, just a server install of Lion.
The built-in AD plug-in does what you need, so you can authenticate logins and share home directories. Then use the Mac server to set OS X policies.
Apple Remote Desktop is now unlimited license from the app store as well, it's a great tool for remote work on the macs, including setting up customised scheduled tasks such as software updates and pushing software to multiple macs without touching each one.
I hope this helps, no doubt I've forgotten something.