Deb
You most likely do not need a pump, but that cannot be determined for sure until you open the floor and see the depth of the line where it goes into the concrete--you need some depth to properly drain, trap, and vent the basement fixtures. n The vent will need to extend up and through the roof by itself or tie in with an existing vent above the flood rim level of the highest fixture. The existing stack cannot be used as the vent for the basement since it receives drainage from an upper floor.
I have to tell you that this is not a good first plumbing project for the DIY person. The tricky part of all this is getting the venting correct. I recommend that (at the very least), you hire a licensed plumber to lay this out for you, show you the correct fittings and where they must go, and have him/her check it after install (but before cover) to make sure that it is correct. If your area requires permits and inspections, do it.
Deb
The Pipewench