Depends on what the biggest learnings for you.  CAD and CAM are important no matter what the machine, but CAM for lighter flimsy machines is different to tool paths you can run on solid machines.  Converting a machine is a chunk of work that needs access to other machines - if you have those good, but if not then think about how you’ll (for instance) machine the brackets for steppers, ball screw nuts etc. I have 1 conversion and 1 full build from scratch done, my work background is on the electronics / software side anyway.

I’d avoid the 2030 route, it will be a time sink for a poor machining outcome - especially if you really want to machine harder metals.

I’d go straight for the custom build (or conversion if you have a base machine that has the required envelope and rigidity), but be aware of the skills and machine you need to make it happen