If I tax-defer self-employment income to an Individual 401k account, are funds subject to self-employment tax when I withdraw them later, or to regular income tax only?

No you can't do that that way.  You still need to file Schedule C for your self employment income and pay the self employment tax on it now.  

The 401K contribution is completely separate from it.   You can contribute to a 401K up to the amount of your Net Profit reduced by 1/2 of the SE tax OR to the max contribution allowed for a 401k whichever is Less.

So when you take it out you only pay regular income tax on it.   

Correct.