Even if the speed dial is broken, the cost of repairing it is not all that great and the mechanical skills required to do the work is minimal.
Tearing into it would be a good way to begin learning the inner workings of the Mark V.
However, if it is not broken, don't fix it.
I don't disagree with 8iowa's recommendation to run the machine through a series of speed changes. I would do that with any machine that I bought.
I wish you the best on your decision to buy.
Here is
some information you might want to browse through to make your self a bit more knowledgeable of the Mark V.