Thanks, M B. I asked the company I rent my VPS from about it and they said it should be fine to add spamassassin with my current setup but ClamAV would be a bit too much. So I've done that. I would need to purchase a VPS with more ram to get ClamAV to run without stalling.
For future reference, I followed the (albeit a bit old) instructions at DigitalOcean to install spamassassin, adjusting key pieces to match Ubuntu 14.04 LTS and current installation scripts.
For future reference, I followed the (albeit a bit old) instructions at DigitalOcean to install spamassassin, adjusting key pieces to match Ubuntu 14.04 LTS and current installation scripts.