What router are you planning to use? The 4x2 aluminium channel (assuming 3.2mm wall thickness/10SWG) weighs in at just on 3kg, and the z-plate is around 2.4kg (based on approx measurements taken off pic) so your gantry, including slides etc, is going to weigh in at 6kg+ plus a router, which typically weigh in at 2 - 3kg if you remove the extraneous gubbins. A typical gantry inc router & cutter would be ~10kg.

But as Jeff says, thats not too big an issue, mass helps (except in acceleration of course). What sort of drive mechanism were you planning for your X-axis?

Welded steel is rigid if suitably braced but care is needed to avoid warping due to heating effects...