ECU issue/replacement

Hello, the engine management light has been coming on and off in my K11 recently and he is running sluggishly. My local garage says he needs the ECU replaced. I've seen them on ebay, but with different numbers. Does this make a difference? Are ones from ebay any good? Is there a better source? Are they coded to the car? Should a specialist fit it?
Sorry for twenty questions, just not sure what to do for the best? :)
I love my car (Henry) hope someone can help!! :)
 
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