Is there a difference between this and a new set of wheels that has a different offset?
Yes, some. What rdass623 said about the geometry change is true whether the change is because of the addition of a 10mm spacer or because you put on a wheel with 10mm less backspace. There is another difference to consider, though, which is that the spacers are NOT part of the wheel, but a different piece of hardware. You want to make sure to get spacer or adapters that are hub-centric on both sides, and not use "universal" pot cast junk that have poor flatness and don't have proper hub bore (and lip, depending on thickness).
I think Eibach and H&R both make spacers and adapters for the 124, but there are so many wheel selections, you're probably better off just finding a wheel you like that's a little wider.
Also, back to the original post, they mentioned REAR, as if they were only going to put spacers on the rear. Doing that is like putting wider tires on only the rear. What WAS balanced handling will turn into UNDERSTEER or PUSH, because the rear tires have more track and can take more cornering force than the front. That handling change can catch you off-guard and send you plowing into something you thought you were going to be able to steer around in an emergency. Whatever you do, you should do in a way that maintains handling balance.
