When using a game pad d-pad setup, I can't select the buy menu from the command pie menu on normal units where the buy menu isn't at a 90 degree angle. An example is the zombie caves mission, where you have a brain robot. I did notice that you can select the AI modes by pushing left and then down multiple times...

Can the buy menu be accessed the same way? Or is there another setup option I can use on the game pad?

Try using a combination of up and left.

Pressing left and tapping up should work, as should pressing up and tapping left. Also pressing both at once may work.