You can use either nickel, gold, or aluminum for evaporation or sputtering on plastic-like materials, for example, PVC or mylar. But you can also use aluminum, nickel, or titanium or whatever as a solid metal diaphragm. The soild metal diaphragms tend to be used on measurement mics. They are usually omni mics which need a high-tuned diaphragm == high tension. The tensions get a bit high, and they are also hard to manufacture, as the diaphragm size goes up. I am not sure of a big sound difference between using gold or nickel to metalize a diaphragm - I've tried both on the same backplate and had no difference that I could tell. Nickel sticks to the mylar much better and easier than gold, unless you have everything really, really, clean and are really lucky they day you run them. That is why sometimes you see people with re-diaphragmed mics with nickel instead of gold. If you saw that other than on my mics, though, it wasn't me that did it. I have only reskinned my own mics.
It is almost impossible to re-use the diaphragm material if you pull it off. If you have a diaphragm glued to the backplate (for example, the M7... or my capsules), it is tensioned then glued. Then the tensioning ring is cut off. If you remove the diaphragm, you have nothing to re-tension it with - plus it will likely stretch where the glue is holding it on, no matter how careful you are. If you have a diaphragm that is clamped but not glued to a retaining ring (some Chinese mics, for example, the Apex 460), it is almost impossible to re-tension it for the same reason - there is nothing to clamp on to. If the diaphragm is glued to a retaining ring, as you flatten the ring out with the clamp screws, the diaphragm re-tensions itself as you tighten the clamping ring down. Often, though, the diaphragm is what screws up on a mic capsule, so that would be replaced.
Likely, if you are getting a re-skinned M7, you will normally get Mylar. PVC is harder to work with (at least it was when I tried making PVC diaphragms), and is not as common. The K47 capsule uses mylar now, as well. I am not sure if they changed to mylar partways through the production years of the U47 but it wouldn't surprise me.
SPA is Stephen Paul - he was one of the mic gurus that pushed diaphragm thicknesses down rather seriously, along with a bunch of other microphone work. He passed away just over a year ago, unfortunately.