Car jack question
Start off assuming zero friction in the screw mechanism, and decide what pitch screw you will use. You should probably stay with the typical screws found in normal scissor jacks to start. Then figure out the gear ratio that you get via the pitch of the screw and how that moves the scissors.
Once you've found the zero-friction torque required, go back and add in the friction term (assuming lubrication on the screw) to see how much that adds to the required torque. Your electric motor should probably also have a gearbox output, I would think.