They're all basically the same chassis. The difference is in the top shutter speed, aperture preview and self-timer. Only the 1000S model offers a self-timer and 1/1000th s top speed. It also 'boasts' an annoying lock on the speed selector knob. OK, it prevents accidentally changing shutter speed, but I find it a pain to use. If you can live with 1/500th top speed, then the M or J(unior) models are just as robust. Two things to check: Make sure the battery cover is in good condition, and that the frame-counter zeroes smartly when the film-holder is removed. Other than that, those old metal M645s are practically indestructible.