With Stuarts as you get all the barstock and fixings included there is the urge to use what you have aid for so BA and ME taps & dies will be needed. Or put the suppled bits in the back of a cupboard and use metric.

Reeves and Blackgates only supply the castings so it is not too hard to substitue sizes, so you can use what you already have

1/16″ becomes 1.5mm and M1.6

3/32″ becomes 2.5mm and M2.5

1/8″ becomes 3mm and M3

5/32″ becomes 4mm and M4

3/16″ becomes 5mm and M5

1/4″ becomes 6mm and M6

Etc

I’ve built from all three of those suppliers and there is not much difference. Good and some bad from all three.

GM needs good sharp tools, can me soft so watch out for chuck jaws marking it or excessive clamping force on the mill. It can also grab drills.

You are unlikely to get hard chilled areas like you do with iron but it can have blow holes or oxide inclusions which are very abrasive and will take the edge off a tool.

Otherwise it machines easily, I tend to use the **GT inserts and carbide on the mill too. This one was all GM castings

Phos bronze, horrible stuff, it shrinks onto the drill and grabs. I use Colphos for boiler fittings, lovely stuff, machines just like brass.

The nature of the material does not dictate the thread standards, within reason.

The old published designs are almost all in Imperial dimensions and use British threads but you can metricate them.