ECM and Immobiliser unit for Maxima QX

[Outlaw 30v6]

My Maxima had a problem with not starting, first i was told the key had lost its code, that didnt work, then they said it was the aerial around the ignition barrel, now ive been told that i need to purchase a new ECM and Immobiliser unit or second hand but each item from the same car. The car starts and runs for about 4 seconds, and then cuts out. Can you give me any advice on the way this could be repaired or where is best to purchase these items. I have been to Nissan and they have quoted me £2370 - parts only!! As you probably know these vehicles arent worth that much money spent on them. Thanks for your time. Lee