>easy as plug and play?
Yes, just make sure THEY HAVE THE SAME ENGINE AND DRIVETRAIN.
>anything else?
Do the usual maintenance, change fluid, why not put a new clutch and flywheel. Change the shifter cable bushings.
if you are not mechanically able to do it or if you are not too sure, I suggest you take it to a mechanic. There's nothing wrong in not knowing how to work on your car.
I'm just now getting into servicing my car. I already know the ins and outs of all the basic stuff (oi changing, tire changing, etc). So the simpler this is the better. My dad is well versed in taking care of this car, so is my older brother.
OP Update: Just pulled transmission from older engine. Going to examine it and see about having it serviced by a mechanic.
Another OP Update: Just figured out the tranny I pulled isn't for the Eclipse GST, can't use it.