Don't get this crap double nali thing.
Get the stock rad.
As the brother above said, they will take your old and adjust the amount when you the buy the new/used rad from them.
Make sure it's in OK condition. All fins should be straight and check for leaks multiple times. Check the cap properly too. You dont want it loose.
If the fan is working properly, DO NOT let them tell you anything otherwise or take it.