If I find a becm with the same serial number will it need coding in

Also. If I buy a kit with the becm ecu key and locks will that need coding too ?

Yes....

I had a problem with my becm not to long ago if your swapping it because something isn't working I.e heaters or something you can buy another becm and just swap the top boards over keeping the original bottom board then you don't need to code any think as the bottom board is what has your immobiliser coding etc on it I got the advise from someone on this site and it was a nice simple fix