who (within reasonable limits) could beat a supes that possesses Reed Richard's intellect, tech, and prep, the will and unyielding defiance of Magneto, and the high street-sense, analytical, and tactical mind of Bruce Wayne?
The same people that could beat him currently. Minus prep, those attributes wouldn't be any more deadly than a pissed off Superman, which happens frequently after he tries his normal boyscout routine.
With prep, who knows. He'd have increased speed but would still essentially just be Reed and Bats. I'm not sure he'd be able to amp himself too far past his current level with prep, because it gets harder at the higher levels.