right now this thread is in the "General discussion" forum... there's an entire
FPV sub-forum where this is definitely the most discussed topic.
I think if you go with a Zenmuse gimbal H3-3D, it will be easier to install since the product was specifically designed to work together.. also better for people around here to help since they are very common. You should be able to find Phantom2 with Zenmuse H3-3D gimbal bundles easily.
You are definitely correct being able to see what the camera sees live is essential to getting good shots.
Choice between goggles or a screen is really personal preference and budget. You can piece together a whole FPV kit with a transmitter, screen, antennas, cables, etc. and still be less than the cost of JUST good goggles... goggles are way more expensive than just a little receiver screen.
there are definitely several different parts that all need to work together to transmit live FPV, I will summarize them here. You will indeed have to open up the Phantom and put some things inside, but I'm going to recommend equipment that is "plug and play" so you don't have to cut/solder any wires.
you will need a transmitter for the Phantom. You should get an "ImmersionRC 600mW"... because of the next item
this item is a pre-made cable for use with the ImmersionRC 600mW, and on the page there is a video showing you exactly how easy it is to install. so check that out
now you have a transmitter linked into the video feed on the Phantom.. you need something to receive the signal on the ground you can see. Like you mentioned before you can choose goggles such as the FatShark goggles, or if you want to go with a screen, just get one with a receiver built in so the screen can natively receive the signal.
Something like this
Finally, you will definitely want better antennas. The antennas that come with screens and transmitters are terrible. Again, you can go less expensive and spend $40 or $50 and get the DJI Cloverleaf antennas or such, or you can get the nicest premium antennas around
like these. That kit has 1 for the transmitter and 2 antennas (an omnidirectional and a long range directional) for the receiver.
I did not shop around for best price or anything, you can do that these are just examples. If you want more info such as antenna recommendations look around the FPV forum, chat with guys in the threads there or even start your own
hope this helps
cheers