So with the codes and an opcom unit will I have any problems divorcing the ECU from the car and the BID ready to sell?
If you disable "Theft Protection" on the original head-unit, then this will keep things simple.
And it should be okay for you to sell. Then the buyer should only need to re-enable theft protection with his own vehicles audio pass number.
If I buy a new EHU that has been correctly divorced will it be simple to marry it to the car and BID?
It is simple to marry to your car, if it has only been "Theft Protection" disabled.
If it has been "RESET", then this can be paired up to your car, however EVERYTHING will need to be programmed into the Head-unit.
If I buy a new EHU that hasn't been divorced but has a radio code will I be able to fit this correctly?
You would need to fit this to your car, then disable theft protection with the donor vehicle audio pass code.
Then re-enable theft protection with your own vehicle's audio pass code
If I buy a new EHU that is sold as "needs coding", what will this in tail and is it possible with opcom?
Sounds like the same thing to me as being totally RESET. - see above.