I am about to buy a house that was built in 1920. I am on a limited budget and we are going to be remodeling the house so I am trying to do EVERYTHING I can myself, and hire out the rest. I am not a plumber by any means but I feel I could tackle small jobs here and there. I was in the crawl space and took some pictures of the existing plumbing. Some of the pipes were crumbling when you touch them; others had cracks in them. I believe that they are drain lines, including a major drain line running to my septic tank. My question is, can I replace them with PVC or ABS pipes of the same diameter? I was told to basically assemble the pipes in the same fashion and lengths as the originals, and when I cut the old ones out, use a fernco coupler and attach the PVC to the cast iron. Should I cut the existing traps and t's out and replace them with the PVC or ABS? I know that I need to maintain certain slopes and such, I am just not sure of the route to take when replaceing these!

\/Main drain pipe leading to septic. About a 4'6" crack down center

\/This one has a crack on the underside of it.\/

Can I cut the fittings off and install new ones? What do you think would be the best route for these as far as materials and where to cut? There are only a couple places that are bad so I dont think I need to have a plumber re-do the entire house.
Thanks for all the help!