Best anti-virus SW for OSX 10.6.8

I have been having all sorts of problems with mail password being changed, user setup be rearranged, slow and unresponsive processing etc. I’ve had a couple of occasions when a Applecare popup said I’ve been infected with malware. I tried to download anti-virus software that would not load - no support from Total AV to get it to load. I once bought American IT support, only to find they were a canadian outfit with a bunch of middle-easterners providing support, and problems were recurring. They wanted a mint.

This MacPro 4,1 is no longer under warranty, and I have an old OSX 10.6.8. I’ve been holding off on an upgrade because the final cut pro application (6.0.6) I use for music videos will not interface with anything more current. Now, I have no choice but to purge malware, upgrade operating system, and look down the road to an upgrade to FinalCut 10.3.2 audio/video editing software and go back to school to learn how to use it. First, I need best option on ridding myself of viruses. Then I need a copy of OSX 10.11.6 to upgrade as far as I can with current hardware. Lastly, I need to load FinalCut 10.3.2, and get up to speed. Can anyone offer advice?