There's so much of this post that screams security issues not to mention potential issues with ISPs.
What kind of bandwidth are you pushing through the connection? Will the computer be used by the general public also, or is it operating a program on the machine that is calling out?
Setting up a computer that is outside of the DMZ is one thing, but when it comes to exposing the broadband used by an individual, if there IS any activity that takes place it's the user that suffers the legal charges, disconnection from teh ISP and potential blacklist from the ISP for future connections.
So, I think more information needs to be disclosed on exactly what the system is being used for. I can think of a number of things from emailing, blackhat SEO, public FTP sites, warez (of course), hacking, dDOS attacks, etc etc. Of course it could be something as simple as SETI
I really don't see why any LEGIT project that you are running is not operating out of a secured data center that can provide the bandwidth and connectivity that you need.
As they say - buyer beware!