I would highly suggest finding a fridge/freezer that is old enough to have metal insides otherwise you'll end up replacing the entire inside and unless you're really good with sheet metal this could be a real bear!
For my vent I just used a piece of 4" aluminum vent pipe. Seems like it works well, although I haven't smoked anything in mine yet. Still seasoning it!
You could certainly swap the controls off you MES but why would you? I used an old oven control board but a lot of the guys build PID controllers which I would certainly use if I hadn't found the oven controller. I would say the element from the MES wouldn't be big enough and I doubt the controller from the MES can handle a much larger element. So I guess what I'm saying is no the MES isn't a good idea, not to mention why would you want to ruin your MES?
There are tons and tons of options for elements. Mine is the broiler element from the same oven I stole the controls from and you can buy broiler elements from all kinds of places online. I know BassPro Shop sells replacement element for the ECBs for $30. I've read of guys using finned strip heaters and again there's all kinds of places you can get them online just Google them, I know Grainger is one source.
You can check out some of the pics on my post for ideas on the vent pipe, element, and controller.
Enjoy and good luck!