Everything you need to know about the computer build is here:

https://www.pugetsystems.com/solutions/content_creation/photo.php

You'll notice that the requirements for Photoshop, which is processor clock and RAM intensive are different from what is needed for Lightroom, which requires less ram, but faster disks and multiple cores on the cpu. This is because when you are editing, you are editing a small preview image, and the only time you really use up system resources is when you are merging HDR or Panoramas, and of course when you are exporting a file - since rendering uses up both ram disk speed/bandwidth.

As far as your specific questions:

1) 16 gb ram is minimal, 32 better, with the option to easily install another 32 is best
2) Discrete video card is a given. Otherwise the memory for graphics display is "borrowed" from the operating system and program execution
3) At the low end, an 8 bit display with a 12 bit LUT is a decent choice - a better display will be a native 10 bit display. the low end costs $300- $600, the better displays >$1300 but they also usually include profiling tools and software to keep the color accurate. All monitors "drift" and age and require regular fine tuning - regardless of whether it cost $150 or $1500.

I'd be suspicious of these guys who "made the change and never looked back" because clearly they are not up to date on the current state of things. In computer time 5 years can be 3-4 generations.

Systems with m.2 NVMe PCIe drives will boot and execute a little faster than m.2 SATA - and you will see more a performance improvement with large files. Does anyone who does photo editing have small files?.