A positioner changes the way you work more than a new power source does. A circumferential weld made in the flat position on a rotating part is even, with no stops for repositioning and no overhead welding, so both quality and speed go up. On repeat production such a station pays for itself in months.
Start from the mass and the position of the centre of gravity. Catalogue capacity applies with the part centred on the table. If the part overhangs the faceplate, capacity can drop by half, which is why manufacturers publish tilting moment charts. Check as well whether the table has a through bore in the spindle and an earth connection through a slip ring, because welding current should not run through the bearings.
With turntables and turning rolls the speed range matters. Circumferential MIG/MAG welds typically run at 0.1 to 1.5 revolutions per minute on diameters of a few hundred millimetres, so the unit needs smooth control at the bottom of the range and torque at low speed.
A robotic cell makes sense on repeat runs with simple welds. Budget for the whole cell: robot, power source with digital communication, positioner, fencing with light curtains and a nozzle cleaner. Tell us the make and model of your power source and we will check whether it has a suitable interface.