Computer equipment lifecycles

I’m currently working on a 3-year plan on computer equipments for my non-profit organization. I’d like to know about how many years for new desktops (60) and laptops (5) would last and about how much these new desktops/laptops would cost me for these 3-years. Would I be better off buying out these new equipments or leasing them? Your advice will be appreciated.

-Keith