Been building for over 20 years

[Contra1971]

but Windows PC (or Linux)
never even used a Mac just iPad, iPod and iPhone
I was thinking about buying a used Mac mini, what should I look for (besides hardware, remember I have been building for many years)
do certain OSX have to be registered and some linked to a user such as iPods? or iPads?

if I buy I dont was it to be linked to an others account and then it is dead to me

but most importantly, at what OSX is capable of iMessages?

 

[Weaselboy]

I would want a mid-2010 model or newer so you can still run the latest macOS version Sierra, and those have Internet recovery built in.

iMessage came out with Mountain Lion 10.8.

With Internet recovery you can just hold command-option-r at boot and get to the recovery utility where you can erase the internal drive and reinstall the OS. That will get you Lion installed then you can update to the newest macOS version Sierra from there free.