Win2k --> firewall --> ipsec /freeswan --> remote network

Hi, I want to set up a freeswan server that will be used basically to
create multiple "road warrior" tunnels from remote Win2k and XP
clients to a local network. I want to put freeswan on a linux machine
inside a local network for other remote machines to access that
network.

I'm somewhat confused about this and have read differing reports about
under what situations this is possible. I have a few questions if
anybody could answer one or more of them. Thanks.

1. Do you need a static IP address for the remote client connection?
2. Will the freeswan server work if it is on a NAT machine, i.e. it's
local address is say 192.168.1.20 which corresponds to a real IP
address that is translated through a firewall/router?
3. What is "opertunistic encryption" and why is it important?
4. Do I need to use opertunistic encryption to accomplish what I want
to accomplish?
5. Will the standard 2k and XP clients work without a lot of
configuration and any 3rd party products.

Thanks.