I have a steel lower, if you are interested.
Will have to compare the attachments but we can come to a deal I am sure.

Honestly, there’s not a lot to do, I prefer a better brake, but the thread limits the choices.
I’m odd, I go back and forth on the fixed vs collapsing stock on these, but with the price of the collapsible being what it is as well as the weight, I’d probably stick to fixed.

Night sight front insert.
Decent sling.

Typically, they like to be a little dirty.
I can’t explain but every HK rifle I have had, liked to be wet (oil) and a couple or more rounds under the belt for utter reliability.

Magazines, remember that the HK mags were thought of as a consumable or disposable. Buy em cheap, stack em deep and when they start having issues, junk the body, especially the aluminum ones.