A good mechanic, experienced with Alfa, with a ramp, would change an engine in a day.We'll say he charged you 50 an hour, so something near 4/500 would cover labour.
If you got an engine for free, thats your cost, not allowing for fluids, filters and any issues you may encounter.
You'd buy an engine for anything between 250 or 500 depending on quality. Either way a belt change would be recommended unless you knew the history of the engine in question.
So expect to spend at least 1000 Euro. If you absolutly love the car, thats the way to go, other than that, move on and buy a runner.