Length isn't a problem even at 1 metre.
You don't move the tube, that stays fixed at the rear of the machine.
The laser fires along a plastic pipe onto a 45 degree mirror mounted on the end of the Y axis, it turns the beam 90 degrees to another 45 mirror on top of the Z then thru the focus lens which in that case is 50mm above the work.

Everything has to be spot on as regards lining up and focus but it's made easier if you build into the lens holders some some of micrometer steering mechanism.

I have been involved in setting up laser machines in the past, that part isn't a worry but we were operating with 3.5 to 5 Kw lasers and 2mm plastic was no problem, in fact 15mm steel wasn't a problem either

What I want to know is what is the minimum power needed for cutting this material.

John S.