I was looking at the other items of the seller in the first link, and he has a full set of cams (minus 22) for what seems like a reasonable price. From his description he seems to know what he is talking about. I didn't know that you can't do double needle or change the stitch width on the 401 when you're using some of the built-in embroidery stitches until I read a post by Miriam about it. But he seems to know about it. It's still a bit confusing to me. (Jan, your 603 might be different.) I haven't really used the embroidery stitches on my 401 except to test a few.
I don't have any affiliation with this seller, but since he seems to know what he is talking about (re slant machines, anyway), I would buy his walking foot because the price seems to be the best and he's got good feedback. (DO shop around though -- I didn't do a thorough search -- I just quickly found a couple examples). I have the same foot (vintage, like this one) and it works well.
I trust the vintage parts more than I do the new parts, but I'm thankful that new parts are available if I need them. I don't have a walking foot for the FW but if I needed one I would probably buy Jenny's (sew-classic) because it is SS and she stands by her merchandise, and it's probably the best price. The original walking foot for the FW is called the "Penguin foot" and if you can find one it costs an arm and a leg!