The problem with normal stepper motors with around 100-200 steps each revolution is, that they got a low maximum speed range - typically you cannot make them run at speeds above 1000 rpm. You also like to be able to sew fast.
No problem in specifying the mechanical properties. I shall do it on the main shaft of the sewing machine, because then the gearing is free to motor.
Speed range: 8 to 2000 rpm. At low speeds you need to provide a short term torque of 3.5 Nm. The motor power loss should be below 350 W in this situation to limit too fast temperature rise (it also depends of the mass of the motor). The mean torque at low speeds is likely below 1.2 Nm. At high speeds a shaft power limit of 140 W is sufficient, and it is 0.7 Nm. At low speeds you need good responsive speed control to limit the effect of load variations.
It will be nice to add on needle stop, that can be disabled and you can choose to stop at needle down or needle up. These positions are within 20 degrees of shaft. You then need to be able to get a fast stop from high speed with the inertia of machine and motor. Normally it is done with a magnet somewhere on shaft and digital hall elements to provide the stop signal. Low cost of components are preferred. Noise from motor is allowed, but it should not be annoying. Sometimes you like to have a little noise, because your ears works as a fast feed back on what is going on with the sewing proces.
I hope this provides reasonable information about requirements - otherwise ask.
With a brushed DC motor I looked at solutions with a gearing of 1:10 to motor.
I have tried to look a bit on the BLDC motors for drones and some RC hobby equipment. The price of such motors are going down. It seems that a lot is going on in development of FOC with ESCs to such motors. The motors seems quite powerful even though the weight are low. But for drones you do not need the low speed control and torque. But similar motors are used for robotics as well.