First off: great move picking up Neugebauer! If he disappoints, no harm done, but if he develops, he could be a big part of your push for the title.
Ok, that was the easy part.
The trade offer is trickier... Erstad and Rolen are both solid, but you won't miss them too much, especially if Ensberg performs well. And any time you can pick up Rivera (I'm assuming Mariano), it's worth seriously considering. Lee is decent, but he may not start much for your team, and Hernandez can be boom or bust. The trouble is that you'd have to drop two players, one in the trade and one to make room for Ensberg.
I don't think there's a clear answer here. Erstad is nice, but of is deep. Rivera would instantly rid you of one of your team's biggest headaches - what to do if Herges, DeJean and Leskanic don't come through for you. On the other hand, you're taking a risk with Ensberg and Hernandez, and Lee won't contribute much to your team.
I guess it comes down to these questions: will Hernandez be able to help you, and can you find a reasonable replacement for Ensberg if he disappoints? I'm not sure about Hernandez, but with any luck, you'll do all right at 3b without Rolen. It's a tough one, but I'd probably accept. First I'd see if there was any way to improve the deal a little, though.
Any other opinions? This one is tough...