I had a person buy some machines from me, and she tested each one of them. She had not used vintage machines for her work but when the motorized Singer 66 sewed through 6 layers of denim she was sold, and she bought 6 machines (3 66s).
You might want to consider one of the Kenmores with the pulley reduction system, i.e. there is an extra pulley between the motor and the hand wheel to basically give the machine extra power especially at slower speeds. I have found 158 series Kenmores having this but recently a 148 series machine also.
Correct gauge needle and correct weight thread are critical.