Nothing wrong with Dexter meat, and the bit of the leg that's shorter is the bit with less meat on anyway!
If you've been offered them, then chances are the people you buy from would be able to help with advice? Maybe even loading when the time comes? Which has a value.
Also, go and see them and get a feel for their temperament before you say yes. If the owner isn't keen on your getting near them, it could be an indicator they aren't as quiet as a first-timer should want.
How old are they? When were they weaned? Will you be keeping them indoors or out?