What to replace on property meant for rental?

I am having a hard time figuring how to estimate rehab costs for a rental property. Obviously you would replace or repair what isnt working but what about outdated things that are still functional?

For example, this house has some old/outdated kitchen cabinets but are still functional.

It also has some knob and tube wiring and mixed with newer wiring.

Again, this is would be a rental property.

thanks