I'm sure you are aware of some of the basic differences:
-speedometer in miles per hour
-wrong color rear blinkers
-no headlight leveling
I'm sure there's more. Just the first things I can think of. But it's basic car stuff, nothing speciual.
How strict are the Portugese authorities about this kind of stuff?
The good news is the electric drivetrain doesn't need any regional adjustments. No emissions! And the US charger will work on European 230V just fine. You will need a "Type 1 Plug" to charge it, which might be a little less common in Europe than the Type 2.
Watch out that most used EDs offered in the US include BAP (i.e. the battery is owned by MB and not included in the price of the car). The sellers typically won't tell you that. Make sure to call MBFS with the cars VIN to make sure you are buying the battery, too!