I told my wife that soon I will find a DIY on YouTube and give it a go. She isn't keen.

Been there... still there...

If you do end up attempting to lay drain lines, just remember... it's a 1/4 inch per foot slope back to the stack.

And, for personal experience, Diablo blades and a sawzall work quite well for cutting through cast iron pipe. But make sure you leave enough pipe left close to the stack to attach your PVC...

And closet bends are a pain to align properly.

And don't set the house on fire with the propane torch.

My house was built in 1928, much of the original piping was leaking when we moved in... the previous owners stuffed insulation into the kitchen ceiling and repainted everything to hide the known issue. 18 months later, the kitchen ceiling was dripping water from completely corroded 100 year old galvanized inlet pipes.