I hate to answer this way:
You probably shouldn't reuse a module.
However, that is not to say that the content of the module isn't helpful.
The primary requirement in this question is to reduce the prep time for a game while making the most use of content.
Modules have two major components: mechanics and story.
In mechanics, fighting the same battle over and over again is bloody boring and is contraindicated by the nature of leveling. There are, however, components of a battle that you can reuse.
- You can steal the maps for new battles
- You can steal the rough configuration of enemies for new battles. At worst, a simple reskinning of the enemies makes it a new battle. I suspect you can get away with this twice. Especially if you have a lenght of time pass, uplevel the reskinned enemies, and give them a new signature move.
- You can steal the rough patterns of encounters
This one is a bit tricky, but if you run with it, the PCs will generally have to make the city their home base. They then can clear out a given scenario once, and after a few levels, a new, bigger, and badder enemy can move in. While just reusing content would get old, reusing the story elements of mechanical components means that the playres can see the effects they're having on their countryside.
- You can reuse the magic items (most magic items released scale)
In general, the weakest area of reuse is the mechanical side, but then it's also the easiest to prep.
On the story side, however, there's a whole new ballgame:
You can base your campaign world on the modules
The modules contain story content, history, and personalities. By using this as the basis of your game, your prep work is significantly reduced. Once you populate an area with known dungeons, history, and NPCs, you can move to a Prep-light philosophy and let the players react. With prep-light and 4e, you just prep a few mechanical encounters beforehand without worrying too much about how the monsters are skinned. Then, when it feels like it's time for an encounter, you drop your encounter in in the appropriate skin of whatever foes the PCs have confronted. Also read this, and this. By using the modules as the content basis of your game, you can "reuse" the story they offer by building on it and thereby significantly reducing your prep.
In no circumstances should you reuse the plot of a module. That's boring (unless played up in a campy way, but that requires very... specific kinds of players).
Essentials characters are valid 4e characters and can be used in every way like 4e characters can. There's no edition difference between essentials and 4e.