Question regarding porting, Best Buy offers, existing VZW accounts and UDP

Hi everyone. Thanks for taking the time to read this.

My wife and I are currently on the ATT UDP with two iPhone 6 phones. My father is on VZW and has a phone with data, a tablet, and a dumb phone (flip phone - no data) on the account. My wife and I would like to join up on his plan, providing it can save us money.

So questions we have... -It appears as if the best way to do this and get nice new devices is to do it at a Best Buy? My wife will be likely upgrading her iPhone and I'll be going to a Samsung device

-The base for the Plan will be 100 access plus 60 (3 phones) plus he costs for the tablet, dumbphone and taxes, right? What charge would the dumb phone be?

-I've read different things in this forum and would like to get a clarification. If I trade in my iPhone 6 for a GS7 edge, the subsidy only covers a part of the monthly cost right? There's still a nominal 5-10/month fee for the device?

Thanks all!