Twin weapons rarely work. I'd stick to a single weapon especially for a first machine.

"Should be legal in all national/international events. If I'm building a war robot, I want to be able to use it."

Stick to FRA rules for the time being. These will see you good throughout Europe. There are some differences state side but I wouldn't worry about those for just now.

"Target budget £1k."

Doable but keep in mind most competitive machines in robots wars cost three times this at least. No machine in the final has cost less than this. Live events are a different kettle of fish.

"Where is the best place to find out component specifications that I can plug into a CAD package?"

From the manufacturer. Ask them for a data sheet. Should give you all the specs you need.

"Things like electric motor size, power/torque output and cost. Wheel sizes, battery pack sizes, what control hardware I need, etc, etc, etc?"

I'd recommend reading the various other threads on the forum. There are so many variables in this question I couldn't possibly begin to start answering it.