Which LEGO Technic sets include instructions to add the Power Functions accessory box?
I recently purchased both 42006 Excavator and 9396 Helicopter, both of which comes with instructions on how to add motors. 

The reviews for 9395-1: Pick-Up Tow Truck suggest it can be motorized.

A lot of the more recent Technic models are either too small for a motor or already come with power fuctions.