IIRC the 400 and 500 series machines use the same tophat style cams as the 600 series. If that's the case a basic set of cams is easily and cheaply available. You may pay a bit more for cams in the correct box for your machine than for the later models but even then they're not hard to get.
I'd go with one of the 500 series machines just for the styling. Make sure they have both doors. Replacing them can get expensive.
I'm not sure if Singer made a monogrammer for the slant shank machines. Maybe someone here can say.
Rodney