PMK has some strict requirements for custom legends. There's a document floating around. I'll see if I can find the link. I know that they only accept SVG files, for example.
Talk to engicoder. He helped me with a couple designs for Danger Zone.
As for interest checks... They will just use whatever art you provide. If you read their web site they say they'll even take crayon on construction paper. I don't know how true that really is but you get the idea. The better and more
accurate your art is, the better your chances of having a successful GB. So use whatever tool you're most comfortable with.
A good 3D render can sell a lot of key caps.