I had to run some 12v wiring on my SM. It's not difficult but will require a bit of patience and a good work light :).
I would definitely recommend that you tap power from a converter and not from the starter battery.
At least in my experience it was a frustrating chase to find a current leak in the starter circuit due to an accessory failure that the previous owner had installed. A dead starter battery is not a high problem, but it is a nuisance.
At any rate, I would keep the starter circuit dedicated and isolated to the one task.
Just my €0.018275 (current exchange rate notwithstanding) worth :
SM 209, Adagio