One best practice is to RESPECT the people who built the systems that are already in place. Think about it, they were able to achieve so much financial success that they were able to hire the best and brightest: YOU

So, respectfully implement the obvious stuff like version control, backups, etc and leave all of the 'can you believe how crappy this is' attitude at home. I'm extrapolating a bit from your 'horrendously designed' and 'zero knowledge' comments. Those people probably still work there, or are thought of fondly by the people who remain!