About to fall down the rabbit hole ??

[JAZZCNC]

When beginning to draw up a design, is it better to find the sizes of linear rail and ballscrew that are readily available and size the frame to those existing components. Or just draw the machine out to the desired size and worry about those after?

Design the machine you want then have the components made to size. It's highly likely because of budget you'll buy Rails and ballscrews from China and they will machine ballscrews to your spec. Same with rails, they'll cut to your exact size or like Neale says just buy little longer than needed and cut your self.
The important part is choosing the correct diameter and pitch to suit your machine. Depending on length you'll also take into consideration end bearing type but don't worry about that yet your still some way off worrying about those things.

Are Nema 23 steppers a good fit for a machine of this size, 1200mm maximum assuming a twin ball screw design on the Y axis?

Yes perfectly fine but need to look closely at the spec because not all steppers are equal and come in various flavors ie: 4 wire 8wire.
Like Neale indicated the inductance rating is important but this rating changes depending how the phases are wired so you'll need to understand this to get full picture. Without getting too technical I'll explain quickly the differences and why one type/setup suits Router better.

The phases of Steppers can be wired in 3 ways depending on how many wires motor come with. These are Bi-polar series, Bi-polar parallel, Uni-polar. With Modern drives only the first two of those are used so won't explain Uni-polar.

Series Wired motors generaly provide high torque but require high voltage to achive high RPM. This is because the inductance is much higher.

Think of inductance like pushing car with sticky brakes and the people pushing being the voltage. The more sticky the brakes the more people needed to push car the same speed.
So lower the inductance the less voltage will be required to achive same speed as high inductance motor, or put another way, with same voltage the motor will spin faster if inductance lower.

So series wired motor are generaly best used for machines like Mills or Lathes which don't require high feed rates(rpm) but do require more torque due to heavy design.
Also because series wired motor uses roughly half the current of Parallel wired motor smaller lower cost drives can be used.

Parallel wired motors are pretty much the opposite. The inductance is half that of series wired so require less folks to push, down side being when going uphill those folks will need to be stronger. In real terms what this means is that double the current will be required when wired parallel.
In performance terms what this allows is that higher RPM can be achieved and they hold the torque further up the RPM range, down side being there is less torque at lower RPM.

Now this is where it gets little confusing to people because motors come in 4 wire and 8 wire(often called Hybrid).
4 wire motors are fixed by the manufacturer and can be wound either series or parallel, often they are series wound.
8 wire motors can be wired in either way by joining wires in certain order. (they can also be wired Uni-polar)

So cutting to the chase for Router builder your better using 8 wire motor with low inductance when wired Bi-polar parallel. This setup when combined with correct voltage(65-70Vdc) gives best performance.

Is there a best practice order of design? Z axis first, gantry (X) first or any other methodology?

Like any good structure it all starts with solid foundations. Design from the bottom up and each will form the other.