The OS .46 AX is hard to beat. If he'd prefer electric, Rimfire makes a .40 equivalent with a recommended battery and electronic speed controller.

That said, the engine will be one of the last things he needs. Building and covering tools will come first, as will all of the hardware and radio gear. It's really neat you want to do this for him, but it may be a few months before he's ready to mount an engine in depending on how busy he is.