I am proud to say that I beat it this evening, most of what I did was run through the missions while cloaked, haha. I'm such a n00b.

It's east, you just die quicker.
Plus if you already went through it on a lower difficulty, you already know whats coming, have tons of attachments, and most of the modules.

So, once I'm done the campaign, can I restart any mission with all the collected attachments + unlocked modules or do they disappear?

You keep em man, you'll breeze through the campaign after completing on a lower difficulty.

I have beaten the game twice on SS and still haven't gotten the achievement... Anyone else have that problem, and more importantly, a solution? It is the only single player achievement that I have left