Bathroom addition with separate vent stack and new line
Allow me to recommend you call a licensed and reputable plumber to come and look at your house and make recommendations.  Surely you can find someone who for $150.00 will come and tell you what you can do.  You could go the sly route and request a quote, you could offer to pay for it, and then ask a lot of questions. Make sure to ask for and get, with your quote a simple drawing and piping lay out.  Good Luck.