The "/M" thing was explained by Erik.
So i'll add my opinion about whether an older version would be worth looking at: yes, it would.
Provided that it has been treated well (which also holds for later versions) the only things are the single layer coating vs multi-layer coating issue, and, well... age.
A wide angle lens benefits most from good antireflection coating. But the difference between T and T* coatings is really not much. But still...
And age is something you can't get around: being older, the chance that it will be worn a bit more than a later model is high. But it doesn't mean that it indeed is in less good shape. And, of course, a later model can be in a worse state than an old thing.
So keeping that in mind, figuring it into the price you are willing to pay, have a look at the condition of the thing. Then see if the price is right.
If both are fine, why not?