I just got a 642 for my wife. The +P thing will take care of itself. The hotter the round, the more it stings. I mistakenly fired what the gun dealer sold to me as "practice rounds". It was Independence brand with no velocity markings or +P indication on the box. Turns out they were nearly .357 level energy and stung my hand so bad, there's no way I would fire enough through the little 642 to damage it. We put a larger Pachmayr grip on it and she carries Speer short barrel Gold Dots. It still jumps in your hand pretty good, but doesn't cause your hand to go numb and make you hesitate to shoot it.
It seems durable enough for occasional +P use, so I don't think its a problem because its really not a fun gun to shoot. Also, only the frame is aluminum. Cylinder, barrel, trigger, and possibly the crane are all stainless. S&W also added a reinforcing "bump" on the left side of the frame before they certified it for +P use, so I guess they know what they're doing. All in all, I'd say the gun can take more abuse than your hand.