Using VPN with IPCOP through another firewall?

OK, it looks like I'm going to be using IPCOP as an internal
firewall/IDS. And VPN server. (I can do that, right? I can't find much
SPECIFIC doc indicating that IPCOP makes for a good VPN server...but I
get the implication that it is.)

Anyway, we're going to have a user on a Windows machine needing to
connect to our network through VPN.
We're behind a firewall/NAT that's provided by our ISP. They control
the ports and whatnot remotely, but we can ask them to forward whatever
we like.

I want to put the IPCOP between that and the switch that all the rest
of our PC's and a fileserver are on.

Now, is it possible for this VPN user to be able to passthrough the
ISP's firewall and connect to the VPN server on the IPCOP to then
access the network?
If so, what should I ask the ISP to change or open or forward on their
router to allow this to happen?

BTW, what in the heck can the client use as a VPN client?? Can he use
OpenVPN to connect to IPCOP?

Thanks!
Liam