The sign in screen is a Windows one that I've seen on other systems so am fairly happy that it works. You get a normal IIS error page when you put the wrong username/password in.
Cached passwords will always be a problem even on secure systems with HTTPS:// pages so not going to worry about that.
It is going to be a fairly select group of people accessing the pages so again that shouldn't be a problem. Just wanted to know if you did something like pressing shift+esc whilst standing on your head it would let you in!