I suppose that these are inches…
Printing 20 in wide with 300 ppi asks for a width of about 6’000 pixels. Any camera delivering at least 24 Mpixels should do. Go to a higher Megapixel camera for easier handling - shots can often not be done in a way to precisely cover 1:1, specially if the negative h/w ratio does not correspond to the camera’s h/w ratio.
If you print at lower resolution, 24 Mpixels should do. 24 Mpixels (FF) correspond to 6K, so the “display on screen” requirement is covered. As for all other requirements, you’ll find lots of hints in the tech section of the forum here and here.