I have a 503 also. I have some cams. No idea which ones. I mostly just straight stitch and zig zag with it so I've never used the cams. I agree with looking at the built in stitches and deciding if you really need the extra cams before spending the money on them.
I have a Bernina from the 90's that has a ton of built in stitches. To tell you the truth, I only use a few of them