My current Mac Mini mid2011 with 8GB of RAM still gives me more than enough performance so I don't need more hardware grunt, but I'm very nervous about there being some spyware/malicious App installed on a Mac from the Apple refurbished store...
You only want official Apple OS releases because of security - I can get that - but you then don't trust Apple's own refurbished machines, which are wiped? And even if you somehow didn't trust Apple, you could always wipe the hard drive yourself and reinstall the OS.