Should I offer less for this new Hail damaged KZ trailer?

I wouldn't worry about an aluminium roof. My Dad's 1978 is still good. As long as you keep the seams sealed, it should be dry for another 30-40yrs.

But if I'm buying a damaged unit, I want a discount commensurate with the damage and that means a discount equivalent to full retail price of replacing the entire roof and any other damage plus a little extra for the hassle factor. (and as others have alluded to it has to be fully insurable)

If you are finding new undamaged units for the same price, it's not even close.