I am using Cecil's patch because of widescreen resolution. Steam version has additional patch with widescreen support but GOG version doesn't. You can play with widescreen resolution anyway but it messes up with weapon's FOV, which causes they are much closer to the corner of the screen and they are not fully visible and I just don't like that. In Serious Sam 2 you can adjust weapon's FOV with console command but unfortunatelly this is not the case for First and Second Encounter so I had to find other way to fix it.
About difficulty I always play on Easy on first playthrough. I want to enjoy the game and explore calmly, not suffer. Not every player is the hardcore one. I remember First and Second Encounter have some differences in amount of enemies depends on difficulty which can mess up with count triggers, but based on my personal experience, these triggers can be set for each difficulty separately so it shouldn't be a problem.