Device from rooted&custom rom -> out of box?

Hi,
Topic is maybe hard to understand but what I mean is that I finally got invite for OPT and i will sell my OPO to friend. She is very basic user so I was thinking to make device looking like you took it off box.

How I can install lasted official rom, delete root access and delete all my stuff from phone so is really empty and ready to new owner

I think unlocked bootloader don't affect OTAs or do it? If it affect then I need lock it back..

Thanks for tips :)