Aaron/gniz has two posts exploring "How do we know a teacher is legit?" and "Why a teacher might not be necessary"
I think it's good to have what, for want of a better term, is a teacher (I'd say "mentor," but that word's been ravaged by business-speak.) I think it's good to know someone who acts differently from you, whose behavior is not as dependent on your behavior.
Of course it is important that the teacher abide by ethical guidelines, and in my way of thinking a teacher who is not so financially dependent on keeping an institution running by soliciting funds from his sangha/congregation is of paramount importance. Ethical considerations follow in such positions that mirror the counselor/client or doctor/patient relationship, and these should be stated and maintained.
Aaron's right: you're going to make most of the progress on your own. I wasn't able to make it all on my own though, and that's why a teacher was highly necessary for me. Luckily, I met a good one; the scam artists out there are more prevalent than they ought to be. But, as I'd written elsewhere, even an ethically challenged teacher might help some students sometimes. But be aware. Caveat emptor.