I'm not sure what you all have in your panel, but I just installed 2 NGT-9000's in my 2 planes, moving up from GTX327 and GTX330's. The upgrade really wasn't all that tough. From a transponder standalone standpoint, it was really easy. Just de-pin the old connector, cut off the pins and put on the included HD sockets, pop them into the new connector and assemble. That's 80% of the way there. Beyond that, add a couple wires for the wifi module, add an AHRS ARINC connection if you wish (optional), and add any serial output lines to your display. For me, it did indeed take maybe 15-20 hours to do everything, but I pulled up wiring from the tail and actually had to open and close the airplane tunnel 2 times to get everything done because I wasn't thinking far enough ahead. But in the end, everything just works.
It really all depends on what you have in your panel already. If you have decent stuff, the effort isn't a big deal.
The G3X is a nice system, so you may want to go that route. But, my opinion is that if you are going to go Garmin, go ALL Garmin and get it all integrated. So gut the panel and start from scratch. I myself would prefer a non-remote transponder, and nothing comes close to the capabilities of the NGT9000, but it really all depends on what you have for all the rest of your avionics. Garmin is good...just not good at playing with everyone else.
