Second hand power on the Canadian National

[Mr Fixit]

I was aware of CN's efforts at purchasing second hand power after seeing numerous photos of the units before and after they were put through the workshops in Illinois, but my question concerns the pros and cons of such an exercise.
What is the likely lifespan of modern high power locos and what price advantage and/or tax/depreciation/insurance/operating benefits are to be gained from buying second hand?
Thoughts and comments most appreciated.
Mark