Then I would do this via VPN instead. Never EVER would i open all ports on the outside of a firewall, even if you could filter on source IP. Spoofing falls into mind.
Instead go with L2TP VPN. It is quite secure, because you use certificates, thereby "you" always know with whom you are "talking" to.
And, if you're doing this with ISA 2004, you can even apply certain firewall rules on VPN Clients from your client.
Cheers
Knutern