I always say get the fastest processor you can afford and as much RAM as you can afford, though RAM isn't that expensive. I would recommend at least 8 GB RAM, but if I was getting something for myself, I'd go for 16.
I have antivirus on my Mac. I've never ever had an issue with a virus, malware, or adware. But, it's really just a matter of time before people start writing viruses for the Mac OS, so I have Norton installed. I don't think it's really essential though.