Re: Why not just a Tor exit?
Both issues apply much more strongly to the proposed box. At least with Tor you get a lot of other people helping to work on the security (improving it, and testing it).
And the metered internet problem is not a significant problem here in the UK, I think. My TalkTalk Business connection is cheap and unmetered. More serious is the slow-down seen by the legitimate users -- but they will just blame their ISP.
Tor is definitely a much better option than an over-complicated, unique design, large (and hot) box.