Would you use used rubber?

A lot depends on the condition of the originals. If they are clean, or can be cleaned, aren't ripped, cut or worn out and are still soft, I'd use them. In a lot of cases the originals are a better fit. I'm not sure of the availability of new gaskets for your car so I'm assuming there are some. In most cases, the problems I've encountered with new gaskets/old glass or visa versa is a difference in the thickness of the channel. Some new glass is thinner and an old gasket won't seal properly in it. Sometimes it's possible to use the old gasket and new zip strip to get the job done. As for cleaning them, I'd get some Toluene at the hardware store and just try rubbing them clean of any glue or dirt. Hope this helps.