the silver discs can all be found on maps that you buy...I didn't do that for the first few levels since they never tell you, so I didn't go super crazy trying to find them all. If you miss one, you are done...no going back.
I also hauled ass and never really died and finished in like 8 1/2 hrs so the under 8 hour achievement may be really hard.
Lastly, the scanning 100 items is glitched, or the description is wrong. It may be scan 100 unique items but I'm pretty sure I scanned everything in the game...well over 300 or 400 times.
Game sucks, have fun playing for over 20 hours!