The toilet gets priority hookup, keep its run straight and without horizontal bends if at all possible - with low-flow toilets paper tends to slow down and clog at bends (personal experience). I hooked up lavs, tubs and washers to the drain with side inlet ells under the toilet to bring in as much water to keep things moving. You'll need to size these properly for the fixture units. In regards to how and where to connect into the 3" stack, that depends on whether your AHJ does/doesn't allow 'wet venting'. Yes, by all means your new bath should be vented properly. As far as using the 2nd floor bath vent, vents are sized to handle a certain number of fixture units. You need to do some inspection and a simple calculation to determine if the existing vent can handle what you ant to add. A good reference for all this sizing and hookup work is Taunton's 'Code Check Plumbing', Amazon has it as well as some of the box stores in their book section.