I have just done this to my wifes 147.
Fitting is easy - you can swap the bad unit for a good one in under 45 mins - it lives under the steering column behind the fuse box. Nice clean job with hardly any tools required - such a change!
One from a scrap yard will be of no use at all - each one is bespoke to the car and is made to suit in Italy. If you fit one from another car, all your electrics will work 100% and the engine will turn over on the key, but it will not start. I bought one from a scrappy to test my 147 for wiring faults as the old body computer burnt out in spectacular fashion. As the scrappy one powered up and didn't burn out after 30 mins of use (car wouldn't start though) I was confident enough to fit my genuine replacement unit. It has now done many 100s of miles with no faults.
Any new one will have to be ordered and manufactured against your cars details and won't need any further coding at all once it gets in the country, but your dealer will try and convince you blind that it will!!! Ignore them.
However, if you have swapped any of the cars other electric components in the past for those from another car (for instance the window switches) then your brand new body computer may realise this, then you may have to get it coded at the dealer (called a proxy alignment - use "search").