If Angle came right out after he won the gold, right now he would be unstoppable. The guy is so intensity and he is a machine. I really don't think there would have been anyone that would have been able to stop him. He would be by far the best wrestler to ever step into MMA. And he wouldn't have been stupid and would have cross-trained.
But as much as I like Kurt Angle and the guy is one of my heros, time has passed him by. If he does choose to do it, I think he would be able to beat a lot of lower tier guys just on Wrestling and Intensity alone. As for the debate if someone could take down Chuck Liddell, well I think with no question Angle would be able to take him down at will, but he would still be KOed.