I'm with Jazz. That machine is little more than a toy. Doubling up the gantry beam will do little for stiffness. Belt drive is for toys, as well as those electronics.

It's main uses will be cutting simple curves and angles in 3-18mm mdf that we can then copy with a normal router....
This makes no sense at all. If you're already making a template with the CNC, why not load more stock on the machine and let it continue to do the work? Especially with curved work, the more parts you can cut on the CNC, the more money you'll save. I've been doing this type of work in a cabinet shop for almost 20 years now.

Spending 2-3 times the money can get you a machine that can do real work. If you want to build a machine to learn, buy a 300mm x 300mm machine.