RE : I have a product key for 8 but not 8.1. I read it can be done with a Win 8 prod key but also read that an 8.1 key is needed.
Win 8 users are entitled to have FREE upgrade to Win 8.1 by going to Windows Store.
RE : After downloading the iso file for Win 8.1, what can I expect to see?
That will not do you any good UNLESS you have a valid 8.1 product key.
You can burn the Win 8.1 ISO file to DVD, but that will be as far as you can go until you have a valid 8.1 product key.