Sounds to me like you need a VPN. First thing you're going to need is some type of Dynamic DNS to setup a hostname to an IP. Im not sure of any, but i would bet someone else on these forums could help you there.

Of what i can see, the 87x series allows VPN's, so you're easiest / most secure way would be to go into the SDM and configure the VPN that way, instead of through the ISA server.

edit: Take a quick look through some of this - http://www.cisco.com/en/US/products/...tion_home.html

Cisco Easy VPN Remote is now available on Cisco 800
I would guess your 877 should have it.