Central Ac / Heat

[Maria Car]

One of my rentals needs a new central ac unit installed , Need some opinions on a new or used

1) New Central  ( 4k)

2) Used Central ( 3 years old ) 2k

Has anyone had experience on having installed a used central ac on there Rentals. The Unit I have now is 20 years old , I got a quote on a new one for 4k and I  also got a quote  on a  used one that is only 3 years old.  Half the price.   Looking for advice if a used one is worth the money if its only 3 years old. I figure it may last 17 years , planning on keeping house for 5 years . I hate to cut corners , I am highly considering it because its a good brand that is only 3 years old. Anyways feedback is appreciated. thanks