If your looking to work your way up start with a 2 axis CNC mill not laser. For one think the laser will cost a lot. You would be better off buying a good desk top CNC mill rather than trying to build your own and buying a laser to go with it.

If you want strength then you'll want to have 2 steppers for your X and Y axis and 2 screw threads. This gives you a sturdier rig and allows you work with heavier materials if the steppers can support it.

The limiting factors of how strong the CNC is will be how powerful the motors are that your using and how sturdy the frame is. You can have the best of the best steppers running everything but if you start to bear down into a material and your frame or screw shafts buckle it won't get the job done. The stronger your overall frame is the more accurate you'll be. You don't want any wiggle room in any of the axis.

Just keep looking at what other people have made and the methods they used to make it. Also look at the way consumer and industrial grade CNC are set up. Copy there frame work.