There are a few missions which give additional rewards if you replay them. Some of these missions give items which do scale with your level. Not every mission is like this and not every item is like this.
The items that do scale have the infinity symbol in the bottom right corner.
To level up quickly and efficiently you should just play through the missions one at a time in the order they are listed in the game. If you ever want to replay a mission that has additional rewards then by all means go ahead, there is nothing wrong with that.
Just keep in mind that the experience you receive from a mission you are replaying is lower than a mission you haven't played yet. For example, if you play a new mission and get 500 experience points then you want to replay it you'll get 200 experience points (I'm just throwing out random numbers here).