I would just say that you've been researching parenting styles and thinking hard about your plans for LO. Say firmly that you've decided that he/she will not be spanked, that you NOW think, despite your past actions, that it is never appropriate to hit a child, and that you will be glad to explain your discipline philosophy if they will listen politely, respect your decision, and not argue. If they don't respect you enough to hear and abide by your wishes, then they don't need to be with LO alone.
You have time on your side too, since LO will not require any sort of discipline except redirection until about age 2...
I think it is beautiful that you want to AP as a SP, and I wish you the very bet of luck!