Yeah exactly the point is not or should not be to get an AI to >= 700 ladder rating.. It should be simple to select, not too much options. Maybe 4 difficulty levels without further options, fixed map pool that is known to work with those AIs somwhat ok, done.
And if you get people to learn to ACU-tank-spam-bum-rush the AI you have already prepared them better for PvP than they were before. Is it going to be boring fast: Yes. But thats where you switch to ladder or TMM if you want the challenge.
Now about ranking them into the 4 difficulty levels: I think this has been suggested before, but the most logical way would be to just let them play in ladder for a time. The question is how to do that automatically.