Start from the part, not the robot
Record the assembly dimensions and weight, weld positions, material, thickness, batch size and expected cycle time. Only then does it make sense to select robot reach, payload and mounting method.
Reach and accessibility
Nominal reach is not the same as usable working space. The torch, welding angle, fixture and possible positioner can significantly change the required cell geometry.
Power source, torch and cooling
For serious production use, check the power source robot interface, wire control, gas, water or air cooling, and the ability to replace consumables.
Seam finding and tracking
Wire touch sensing, edge finding, laser seam tracking and path correction are different levels of functionality. The need for them depends on part tolerances and fixture repeatability.
External axes and positioners
For larger or geometrically complex assemblies, support for external axes can be crucial. A positioner often brings more benefit than simply choosing a robot with longer reach.