I am looking to build a unit with around a 24" x 24" x 24" ( 600 x 600 x 600mm) build volume. Which one of the three would be the best starting point?
Hercules !!!
(sorry HercuLien, in my opinion 
)
Herculien
@William_Eades for 24" cubed build volume I would start looking beyond round rods for the axis. Runout and deflection will be your enemy on larger and larger spans. This is because the rods act to both power transmission (rotation) and an axis guide (translation). I would look into linear guide rails for the sides, then slave rods to tie the two sides together. Look into @Tim_Rastall and his newer ideas.
@Eclsnowman I had been looking at various formats for the build and had planned on guide rails because of the induced harmonics and flexion in the long rod spans. That said, I still really like the straight forward simplicity of your design.
@William_Eades thank you. I have been happy with the design. I am excited to see what you come up with.
@Eclsnowman Being an old lab rat it will probably be an over-engineered nightmare…but it will be accurate! ;-}
@William_Eades I learned the same thing from my dad “if you can’t drop it, and still have it work perfectly… Then it needs to be built stronger”