Visiting the admin office while visiting a school is the logical place to start with setting up a formal tour. If you are just driving by on your own and walking around, you might still want to stop in and ask any questions you might have about the campus.
As far as the stat review goes, if you have not applied at all yet, there is no harm in asking the undergrad liason to review your grades, experience, etc to see if you are on the right track. If you currently have an application in at the school you are visiting, leave it for after the application cycle. Asking for a review in the middle of the process probably won't look bad, but it won't put you that much ahead of the game. If you do want to get a review I would definitely suggest making an appointment with the appropriate person, and this may have to wait until after application season, due to sheer busyness.
As far as auditing classes....I'm guessing from context that you mean having a school review your classes to make sure they accept all the prerequisites you have taken. This is never a bad idea, but again, if you haven't actually applied yet, it would be polite to do this after the schools application period is over. And most schools are happy to do this if you just contact the admin office and have them bump you to the appropriate person.