as m_c suggests..

a 1m lever need 1Nm of holding torque per 100g of mass it has to hold up (including the mass of the arm). So for an arm able to lift and hold 50kg you'd need 500Nm of torque. On a 2.5Nm motor that would require a 200:1 gearbox. Assume the arm has 90deg of travel, that would require 10000 steps, about 5 - 10sec of movement... a bigger motor would need less gearing so could move faster (excluding inertia effects, which I have left out of the calc but are significant as well).

Could be done with steppers and screws, steppers and gearing, servos (motors with feedback), hydraulics, etc... will probably need some sort of positional feedback whatever drive mechanism is used...