I don't mean to be insulting, but it's clear from the questions you pose that you don't have the knowledge or experience to do this without it becoming a potential disaster. Some of your questions can't even be answered without an on-site inspection of what you are dealing with. Do yourself a favor and get a plumber in to at least assess the situation, or a friend who is knowledgeable about plumbing techniques, material choices, and waste and vent lines. What you want to do might be relatively easy, or it may involve jackhammering the floor for starters. And, unless you are living someplace out in the boonies where there are no codes, it's highly likely that you will need to get a plumbing permit from your local building department.

My reaction was the same as kudzu's.

Call a plumber.