Planning my first CNC machine

[m_c]

I have seen a similar design in the past (pretty sure it was actually on here somewhere).

It does work, but the Z-axis is a lot of work to get everything spaced correctly, and restricts access to the spindle.

My concern would be the twin stepper setup, as ideally for twin steppers, you'd want to be able to home them independently which requires a certain amount of flex in the design. With too rigid a setup, there is a risk the motors might end up fighting each other.
I'd probably just go for a single Z-axis screw, as if the gantry is stiff enough, racking under load shouldn't be a problem, especially on a Z-axis.
The biggest load on a Z-axis is going to be doing things like drilling, where accuracy isn't a major concern. The bigger concern is rigidity in the X and Y axes, which twin screws isn't going to benefit.

Generally moving gantry designs are preferred as they take up less space for any given table size, but for compact machines, it's not so much of an issue.