The Poly is just a monorail ride from the MK. I like to go early to walk around a bit before checking in for reservations.
I'm not sure the exact minutes but shouldn't take more then 15 minutes to get there from MK.
It will stop at the Contemporary, and the Transportation and Ticket Center before heading to the Poly.
Your reservation time will depend on what time Wishes is that day.
All the spots on the beach are perfect for viewing the fireworks. It's directly across the lake.
If you make your reservation for 1 1/2 to 2 hrs before Wishes that should be good.
When you are done eating you can explore the Poly if you haven't before.