"I'm looking at installing an all-in-one PC with a 19' screen at the main console and running OpenCPN or similar software on it. Cost is between $400 and $800 depending on the PC you buy. Has anyone else done this?"
I have an onboard PC, not all in one though. I have a marine monitor made by Big Bay which is waterproof and very bright when needed in the sunlight. I run Coastal Explorer, which comes with 2 licenses so I installed it on my laptop as well that I use at home to plan routes. On each machine I pointed CE to read and store routes, waypoints, etc., to the same dropbox/cloud folder so whether the route is created at home or I change/create it while underway, there is only one file and it gets updated each time I save it or when the program closes. The next time I open CE, whether at home or on the boat, it is always up to date with any changes or additions. No need for updating one or the other with a flash drive. The onboard PC is networked to my Furuno GPS, depth, water temp, and heading sensor. I have an AIS receiver plugged into it as well. The only function it doesn't have is radar, which the Furuno handles well anyway.