Hi and welcome to the forums
1 and 3 - The IRIS+ is a great option for this as it includes everything you need for a fully functional drone. It comes with the pixhawk autpilot, and GPS and also the telemtry kit (for wireless mission planning and monitoring). The only thing you need to add is a gimbal for your gopro.
2 - I suggest you order the Live View kit for the IRIS (will be added to our site soon) This includes an FPV monitor, and transmitter to send the real time video from your gopro back to the ground. This Live view kit includes all the parts you need to add it to your IRIS+, including the antennas.
4 - If you are looking for a 3 Axis gimbal for your IRIS+ then I suggest you consider teh FY-G3 gopro gimbal along with this IRIS+ adapter. This is more of a DIY addon to your IRIS+ but we will soon have a cable kit so you dont need to make/use any special cables to connect this. However the offical gimbal for the IRIS+ is the T-2D gimbal which works great, but its only a 2 axis gimbal.