Is the smart capable of a 120 miles one-way daily commute, the answer is yes. Is it my preference, the answer is probably no.
Especially when you will be cruising at 75MPH, you are not getting much power reserve for passing or merging. 0-70 is 19 seconds in a smart, with a 90MPH top speed.
I notice you own a 2002 Cooper, if I were you, I would trade it for a new Cooper S. With a 140MPH top speed, 0-70 in 8.3 second, and factory cruise control option, it will make the commute far more entertaining.
Especially when you are stuck behind slow vehicles and would like to speed up and merge into the faster left lane to go around the car in front of you. Let's say the car in front of you is doing 50MPH, the 50-70 acceleration is over 9 seconds in a smart, where the Cooper S can do it in 3.5 seconds. You will have to find a huge gap in the left lane before you can pull over and pass without the guy behind you honking and flashing the brights.
Then, there is the long hot humid summer in the south. None of these numbers includes the drag the A/C puts on the car, which is significant for low power cars.
I know you may not want to hear that from a forum of smart enthusiasts, but your usage pattern does put the smart beyond its sweet spot.
Now, if 75MPH is the speed limit in your state, most people are probably cruising along at 80-85 MPH. Which would make the matter even less favorable.
If I were you, I would test drive the smart in the summer with full A/C running and see how it handles the higher speed long distance commute you described.