I had a couple of HK91's, basically the same thing.
If you reload, youre going to want to get a port buffer. This family of guns is pretty brutal on the brass. Without the buffer, the case slams into the rear edge of the ejection port, and usually leaves a heavy dent in the case. The flute marks are cosmetic, and nothing to worry about. The buffer stops the dent, and redirects the brass more forward.
Personally, Id leave the trigger alone. They arent bad once you get used to them. I know a lot of people think otherwise though. If you shoot a revolver DA, youre golden. If your a SA fan..... learn to shoot DA.
Once you figure out the sights, I think youre going to like them. They are a little different, but I personally think they are the best "combat" sight going.
The big notch and the "globe" are your CQB/100m sight, you use the whole notch and the globe as the CQB sight, you use the small "V" in the base of the notch and the "post" in the globe for precise aiming out to 100m.
If you take a quick sight using the notch and globe, and then slowly roll your eye down to the small "V" and post, youll see they usually coincide.
The other numbered apertures are self explanatory.
If youre going to go the claw mount route, the ARMS mount is a better choice over the original HK type mounts, and about half the price. They hold zero like the factory mounts, but allow you to mount pretty much anything.
I always liked the HK web slings over most of the others.
Skip the sliding stock.
If you can find an oversize or winter charging handle knob, they are a lot easier to get a hold of than the standard "flat" type.