Any scanner will do. I use a Canon MG5550 desktop scannerprinterfaxcopierpaperweight thingy, and it works just fine. The artwork output for your plotter will be vector data, so if you are tracing over a scanned image, it isn't the scanned image that ends up defining the stencil, it is how cleanly you have drawn the traced vector image. Which is why it is so important to get to understand the vector drawing tools.
As usual, a bit of effort is required, but once you understand how to draw accurately, you will probably find you can do pretty much what you need from scratch. OK, tracing over a bit map might help with some things, but making sure stroke widths on lettering will be vitally important, and you will NEVER trace a bitmap that accurately!
And also once you can draw accurately, and you realise the output is EXACTLY the same as that needed for a laser cutter, a whole new World of possibility opens up!!