What is oldest desktop Mac I can get and still have NIS work easily?

I have a very old PC and since XP is going kaput, this seems like a good time to finally switch to Macs.

 

My friend has a 2008 iMac that still works fine, so I am thinking of looking for one.

 

However, I am worried that NIS might not work so well on an older machine, and plus I think Apple OSs also don't work so well on the old dual-whatsit processors (or something like that).

 

I am not a computer person and I don't even like to uninstall/install things.

 

1)   Is this switching to Mac a bad idea?  I don't really see a choice, since I like things that work without me having to tinker.  I like that I can just haul the thing into the store if things get ugly.

 

2)  How old an iMac does it make sense to look for, given that I would like to use NIS on it (I assume that even Macs need antivirus now, correct?)  and I don't like to have to mess with things?

 

Sorry for the long message.