A couple of things about the JY motor. $250 seems pretty cheap for a motor & tranny. Is this for a NA(non-supercharged) motor or a SC motor? Most important, get the VIN from the car it came out of, the JY should have this on file if it'* already out of the vehicle. Run the VIN through CarFax to verify the mileage. Is the jy offering a warranty on the motor? Make sure you can get your money back or a swap if the motor ***** the bed shortly after installing it.
Okay, now if your swapping from a NA to a SC motor will have to swap out the entire drivetrain. That means everything that'* connected from the motor to the drive wheels. Plus replacing the wiring harness.
However, if your swapping from a series 1 NA motor to a series 2 NA motor you only need to change the motor and do some wiring.
I've done 2 JY motors in my car this past year (thanks to the help of BillBoost) and I can tell you the most important thing is to verify what you're getting through an independent source. (CarFax). And secondly, try to get a motor that came out of a Bonneville. This will greatly reduce your swap time by reducing the amount of bracket swapping you will have to do. (also a lesson learned the hard way)