Hi Ed,
A welcome email is an excellent idea. I would definitely tell new users to have a good look round the site to familiarise themselves with how it "works" before starting their first thread. I would also mention that they should read the
Site Rules and point out to them where they can be found... ;-)
I would further mention that on AB, as on every other similar site, there will inevitably be disagreements and spats - it's human nature - and they may find themselves on the receiving end, but they should not take this personally; reassure them that the site is moderated and that such posts will be removed very quickly.