Plumbing questions

I have a 2 family home It was built in the '50s. I am doing some repairs and updating. It was built pretty poorly in many ways but everything works and has worked for the nearly 40 years I've owned it and lived here with little or no problems. The home is a duplex style. Each side is identical and a 2 story with bathrooms on the second floor. There is one 4" cast iron drain for the 2 bathrooms. The drain for all the fixtures is horizontal and runs about 5 feet max to a vertical section and that has a 4" vent. I have found after removing shhetrock in the unit presently vacant......that the toilets have lead bends and are in bad shape.......there is very little room to adapt to PVC as the toilet is very close to the horizontal run.......The horizontal section is pitched in the wrong direction! Toilets work just fine and always have but water does not drain at the closet bend. I could just try to adapt PVC to the cast iron at the hub or cut it and use a mission clamp and ignore the pitch issue. The cast iron is all ok and there is no leaks and never has been. I could also remove all the cast iron and re do it in PVC with correct pitch. That is a lot tougher job as I am living in one side. I'd have to remove some of the ceiling in my kitchen to access under the plumbing on my side of the house. Also remove the toilet for acces to remove the lead and re do it in PVC. I also have another issue....I have washer drains in the basement seperated by a block wall, Both Tee into one single 1-1/2" drin that is right in the 4" just before it exits the home......I am going to cut the cast iron and adapt to a 2" PVC plumbed as it should be. Do I need to have 2 seperate 2" drains into the 4" or can I tee both washers into 1 as it has been for years? I already removed all the galvanized drain piping in most of the house and redid it in PVC.....I know thats a long winded question but figured maybe someone would have a couple answers to help a seasoned do it yourselfer. I am a mecahinc and do everything I can myself. I know it's a big project if i choose to replace the cast iron......I do have a bathroom I could use for a few days...in my Lance truck camper.....kind of like a porta-potty----on wheels. I know I am a little crazy to even think about doing this....but I know I can do it.....I'm very skilled at every thing I do

Seems like you're pretty set on what you plan to do and I have no advice on running your new pipes, but I just want to say that you need to support the cast stack if you plan on cutting it. You don't want that crashing down on top of you or anything else.

It sounds to me like your best course of action would be to tear it all out and do it in PVC. It's a lot of work but since you will be covering it all up again the last thing you would want is to have to fix fomething else 5 years down the road.

Skill is fine and dandy. Experience is altogether something else.

I am "on the fence" so to speak about doing this myself. I know it's a big job and also know experience is the best teacher so I have put this job off for the winter. The present horizontal run of cast iron is poorly supported as it is. The framing was hacked up for the plumbing. I already removed the lav and tub drains to allow me to do some repairs to that framing, There is no vents for either of those two fixtures. I can add a vent for those two either with a Studor or a seperate vent up thru the roof. Once I opened up the walls and removed the toilet on the vacant side.....I realized this was not going to be easy. I have read some plumbing books, bought a code check book and read a lot on this and other plumbing forums. I would just use a plumber, but I have concerns as to where this project could go once a professional was involved. I could also just do the minimum and cover it all up as I plan to sell the place in a few years but that is just not appealing to me.

lead bends

My advice as to the bends would be to have a plumber remove them and the brass wiping ferrules and install new cast iron bends. For a capable plumber it would be a simple job, and much better for you.