Presumably you’re looking to cut wood or plastics at that size nothing harder?
Are you concerned with cutting speed or high accuracy?
The Shapeoko gains a lot of benefit from the oversized extrusions with their excellent rigidity, from some other threads it takes a pretty chunky cross section of extrusion to stay reasonably rigid (second moment of intertia). The deflection goes up with the square of the beam length. There’s some good numbers for this, see the deflection maths sections in these posts
If you’re going to build basically from scratch on new extrusions I’d seriously consider some cheap ebay / AliExpress HGR15 rails for the X axis instead of the V wheels.
The belts are less of a problem, deflection is closer to linear with length and you can use the kevlar cored belts easily on the standard motors and pulleys.
At this size I’d suggest using a torsion box for the base to eliminate the spoilboard bouncy castle problems too.