I cannot help with any parts, but I can add a few words on the use of 1 or more carriages/slides. Depending on the use of your machine (-laser-engraving is very different from milling in terms of forces and torques applied!) you may be fine with 3 slides (2 on 1 rail and just 1 on the opposite rail), as this configuation is the least overconstaint. If your slides and rails have no play (pretension or prestressed balls are present in the slides), any "out of paralellism" will generate HUGE forces on the balls and may permanently damage them, your slides and/or your rails, depending which part is winning. If you intend to use only 2 slides (1 on each rail), you design is very prone to tipping/rocking around the line which runs from one slide across your machine to the other slide. If your slides are short, the balls on the end of the slide will have to cough up huge forces if you accelerate and decellerate the structure with the ball screw. Technically, it is not a challenge to design something that will last 2 demo's (1 low speed and 1 high speed for friends and family), but you probably want to build something to last, right..?
So depending on your design (materials used, dimensions, stifnesses, etc.) and the accelerations/decellerations you may get away with using only 1 slide on each rail, but I strongly encourage you to make some calculations and look into some specs (especially torque ratings) of some suppliers before getting disappointed (sooner or later).
Just my $ 0.02.
Andre from The Netherlands