Either or will work for the wash. I tend to use something like Agrax Earthshade/Devlan Mud in place of black on Ork weapons. Gives the same kind of shading, but gives that dirty, oily look.
The Ork looks great! I've always thought the green rim on bases, particularly bases that aren't green on top, look...off. I'd either paint the rim the base color brown you used for the top, or paint the rim black. I personally think black rims on bases look best regardless of the basing type, but the green definitely doesn't work with the wasteland top.
Looking great, I would agree with a brown wash over the metal and because orks aren't likely to build all of their weapons from the same metals or use the same gun oils you can get away with different browns on the different models and it won't look out of place. I tend to mix different brown washes and inks in with my skin washes to get some instant skin tone variations without too much fuss.
As far as the basing goes I don't think there is a problem with doing the rims green but then again I use the base rims as my method of identifying the different units. It can get confusing when you have four units of thirty boys running around on the board and I found the bases were the ideal way of differentiating different squads when the battle starts getting hectic.
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