The house is in a rural area and has a septic so there should not be any cast iron waistlines everything would be pvc, which could also crack but shouldn't if it is sloped correctly.

If there is any cast iron I typically replace it since it often has a problem. 

On a 75 year old home, I'd replace any cast iron drain lines that are exposed and accessible.

If you suspect any cast iron drain lines to be original, I would have them scoped - or simply budget to replace them - as they are nearing end-of-life.On the electrical front, you will be looking for active knob-and-tube wiring.  

Examples of this include when a plumbing inspector sees any cast iron left above the main stack they will make a plumber replace all that cast with PVC and possibly dig into the venting, etc.   

This would include things like the shower fixture, any cast iron drains, and potentially even the tub (although I am always tempted to keep the old tubs).