Properly Updating Drains for Basement Bathroom Update
I am gathering that you are going to frame behind the toilet/sink.   If not, this is a no-brainer.   You want to use 2x6 so it is easy to move any plumbing through this (vents).   

Also if I were you - no way in the world would I lose a toilet.   You bought the house with a full bath in the basement and if you take out the toilet it probably doesn't meet the real estate definition of a half bath.   

Rules - 


You need venting.   There are different ways you can do this but I think with new framing you can probably get to your main stack pretty easily.   If you have specifics I can walk you through options.
If you break the ground you are supposed to get permits in most areas.   Would I?  Depends were I am at.   This could also be an issue with removing toilet - your city could notice that your house lost a bathroom or at least 1/2.   
You have already broken part of the ground - get all the metal out of there to your main stack and tie it in with PVC.   


What would I do?   I have done a lot of basement/laundry combos.   


Frame that back wall in 2x6.
Install a deep basin sink with a huge counter right next to toilet.
Put washer and dryer under counter to right of the sink.   
I would install shower where the head is against wall (so your counter wall to the right is the first 20" of the shower.   
I would tie in the shower and sink to toilet after its trap and send everything down this way.   This serves your needs plus the water from shower/sink/laundry flush out any toilet debris.
I would either put a knee wall on left side of sink to hide toilet or a full wall and door for toilet if you want to hide it.


If you can give me the dimensions you are working with I can help layout what you want to do.