[jnaas2]

I cant see any problems with your plan but the one thing I would check is to see if you can legally lay a vent line on its side in the county you live in. The 2 inch line that was originally installed was more then likely a wet vent for the toilet. This is a drain that will recieve water from a sink and also vent the toilet at the same time, the state I live in will not allow a vent line to be run horizontally until it is 6 inches above the flood level of the fixture it serves but it can be installed at a 45 degree anle to horizontal. The line going to the shower may need to angle towards the wall that the sink is located on so the vent doesnt need to be run horizontally. The reason that a vent isnt supposed to be run horizontally is if the fixture drain clogges up it will fill the vent with water and other things and can eventually clog the vent line. One other thing to think of is the drains are run then the concrete floor is poured and then the shower is set, you need to get the rough in dimensions for the shower you are going to use and run the line to the shower with the glued p trap located exactly where you need it. To make things easier you can build a box 12 inches by 12 inches where the shower drain will be and and pour the concrete except for this area and test fit the shower later and have a little wiggle room for the drain line