If you want a loud computer that consumes ten times as much electricity as it should, then an old Xean Workstation might be for you. Personally I prefer buying old Thinkpads. They're cheap, fast enough, and their electricity consumption is decent.
My main objection to the Z4xx series is that the cases are so bulky. I prefer the Z2xx small form factor models, or even better the Z2 (fastest computer I own, but also one of the smallest).
One of the big advantages of a Xeon is that it will use ECC memory, and thus be more reliable.
I never felt a need to measure how much my employer pays for electricity.
disable lid sleep (well, disable sleep entirely on osx 10+ because apple don't want to give you the option to disable lid sleep only) and it will never pass 4h.
the 6/8/10h some people claim is because they don't realize the computer is sleeping most of the time. I guess it is fine if it's just your facebook terminal...
Note the Z420 is a workstation not a server. Buying an old server is completely different.
Look at ZBook, Precision workstations for the best performance for your dollar.
I’d love to get an old one and stick an ssd in there for basic usage.
If you’re in a large(ish) city I’d expect you’d be able to find a similar company. I was able to pick up a x240 in great shape for $125, and test drive it in person before I paid.