Where are you in Canada. I'm in southwestern ontario. I have a large router that would be perfect for a plasma conversion. Alot less $$ and easier to buy from me and fix it up as a plasma than to build from scratch. If you're in the neighborhood, send me a PM.
As to your question for a gantry that size, 125lbs is really not much. Fine for a plasma, but on the light side for a router. No, I wouldn't worry about it.
I clicked on the link. They don't give any relevant info that I could find. Not even how many watts the servos are. Not even the gear ratio of the reducers is given!
Really, the long answer to your question is that I would need to see the torque vs speed graph for the servos, the inertia of the motor, the gear ratio, the inertia of the reducer, the effective radius of the pinion, (assuming rack and pinion), and the all up weight of the gantry including everything that moves back and forth including motors, Y axis, Z axis, everything attached to it. And then I would need to know your goals for speed and acceleration.
The short answer is don't worry about it.
I've heard good things about candcnc torch height controls, but I have to wonder if this servo system is overkill. Nema 34's should be fine with a 2:1 or 3:1 belt driven gear reduction even for a gantry 3 times your weight.