AI buys all of the above, it does not however buy a completely predictable (SP2,3) type of buy.
You can see the AI pick, as is usual, by setting up a game as your opponent with the desired points level (based on your core+support points level say) etc, at the requisite date in GameOptions and in the in-game preferences. (To make the following easier - you will then only need to remember to select computer buy for each test game run).
Now, as usual for testing the AI pick (and as described in Mobhack help AI pick list section IIRC), set up a game with the
computer set to buy the player 1 forces, so you get an AI buy, then examine what it bought in the deployment.
Now - after examining this purchase - exit game.
Now - repeat the computer test buy process at least 20-30 times to get an idea of the OPFOR variations for a particular nation/battle type/point value/date combination.
You will see PIV, kraftrad, and panzergrenadiers (esp if points are high) - both truck and halftrack versions. Nebelwerfers sometimes, and sometimes not, sometimes it will buy ATG (even in an advance) to screen the rear area - which can be rather nasty if the GE AI buys a few 88s in good visibility, and places these in a commanding position. As I learned to my cost when I lost a platoon of Russian Valentine-57mm I had left out in the open when delaying - popped off at some pesky 251s and scout cars and then suddely whack-boom, whack-boom as the 88s covering the GE advance killed them all at 40 odds hexes
!
Cheers
Andy