|What's a good enough level of expertise to code for you own business?

I think it's very much depends on your goals and tools.

For example, you can probably design a useful business application in Mircosoft's lightswitch in a short time .And it would probably be reasonably secure ,maintainable etc. , because of MS's help.

Same can be said for using python + numpy as a front end for a lab equipment , used in your business.