The problem with sewer is

the horizontal pipes do not have enough grade so that sewer stays inside and make chemical reactions...

If you want to keep the house you probably want to replace all the horizontal cast iron 4 inches. The PVC pipes are pretty cheap... if you change all you will probably spend 100 dollars for pipes and connectors. Putting new ones is not problem, problem is to cut old cast iron...

Since you are doing the sewer, why not replace the leaking coppers.. they can only get worse. Another couple of hundred dollars...