The good news is that if your drain truly does exit the house in the floor, then you might be able to avoid having to use an ejector pump (which is used in many homes where the drain exits above the basement floor).
However, the drain lines for your fixtures require a certain amount of pitch. For any shower or toilet where the source is already below the floor, this implies a certain depth requirement of the connection to the main drain.
You won't know how far down that drain goes without opening the floor. Opening the floor around the drain might damage the drain and require it to be repaired.
All this is to say that unless yr comfortable with a demo/jackhammer, understand yr drain materials and how to make repairs and connections, and understand how far yr fixtures can/must be from the drain, then it might be beyond you.
That being said, if yr patient, you can learn a great deal by getting some basic plumbing books from a bigbox store, as well as a copy of general plumbing codes (The Codecheck series is pretty good). THEN, you can sketch out designs and post them here for critique.
At the very least, you will be able to have a very educated discussion with a plumber if u still decide it's all beyond you.