Beam - I have used both open and closed face helmets. It really depends on what you are buying it for. If it is for autox, an open face is really great for visibility. If you ever plan on doing a track day, a closed face is usually a requirement, so I went ahead and bought a closed face to use for both. I don't have it anymore (it was in my car when it was stolen (just got back from a track day at Streets of Willow) so I am in the market for another full/closed face helmet.
As far as harnesses, I wouldn't recomend running one on a street driven car unless you have a roll cage or bar. Just in case of a roll, you would really maximize the chances of a head injury if you were harnessed in without roll protection. A couple companies make harness bars which are a safe anchor for a harness, but don't offer roll protection. Lokar is one of the companies that comes to mind.
In any case, I have a really effective harness in my miata (attached to my rollbar)- you are welcome to come by and try it as a starting point.
nk