Welcome!
Smart is more difficult because there isn't much capacity (space or weight) for batteries. However if you're only looking for a short-range runabout, it's not as much of an issue. Also, is a manual gearbox available in the Smart? Here it's only an automated manual, and that makes doing a conversion more difficult.
Biggest advice: don't get a piece of junk donor car. Even if the price is right, you will just end up with a piece of junk EV.
Ask me how I know this.