I was on the fence between the D750 and the Z6 and went for the Z6.
Both allow you to use old Nikkor lenses (of which I have many), but the Z6 has in-body image stabilization, so even the old manual lenses benefit from that technology.
I also second Tamara's suggestion to check out KEH.com , I have used them for many years for used equipment and they have always been great. They have accurately rated the condition of the items they sell. Never needed to return anything.