I'd imagine someone (even in California) can only "gift" you that which they own. He doesn't really own the house you're talking about, and won't, until that $400,000 is paid off.
If I buy it from my dad, is the lowest I can buy the house for 400,000?
If you want it free and clear of all encumbrances, that would be about right, PLUS closing costs.
It's dangerous to start servicing the bills of another person because that can be interepreted legally as also sharing the responsibility, and that would not be wise without first consulting an Attorney.
But I'm 3000 miles away, so I'll suggest you wait to get an opinion from someone else in California.