Single Player - Replaying Missions

I know you can access previously played missions from the mission archive on the bridge. This is useful for replaying missions on different difficulty levels (and to get achievements) but I was wondering if you gain additional credits and research. On some missions I neglected to collect all extra research stuff and want to know if I missed my opportunity to get them.

You can go back to get anything you missed, but you can't farm levels for cash or research.

So I can get research that I missed and it still counts? That's all I needed really. Sometimes I rush through a mission because I don't have much time or whatever so that is good to know I can go back and get them.

I wouldn't worry about going back for missed research, because there's more than enough to finish researching before the end of the campaign. Without trying too hard, I had maxed out my research about 2/3rds of the way through. Once you hit max, any additional research points are translated into 10 000 credits each.

I am a completionist though, so I cant exactly leave it in good conciousness. Screw getting it all on hard or brutal though, that's just silly.

On the same note, is it possible to get the "buy all of " whatever achievements on one playthrough? I would think that it would've been smart to get the full cash reward for a mission the first time on any difficulty, and then get a small amount for the each of the other difficulties. 

" I am a completionist though, so I cant exactly leave it in good conciousness. Screw getting it all on hard or brutal though, that's just silly. "