[Andrew]

e-clips don't usually have the same retaining force as snap rings.

If you plan to use snap rings, get a special tool (available at auto parts suppliers) for removal and installation.

Don't use either snap rings or e-clips when large axial forces are expected (as in a worm&gear drive).

Spacers are an excellent and potentially high precision method for retaining gears in place.

An alternative to snap rings, if you don't mind the imprecision, is cotter pins (including hairpin cotter pins). You just need to drill the hole in the shaft through which they will fit. You might also find that a clevis pin and a cotter pin can serve as both axle and retainer.

If you put a shoulder on your shafts, you might accomplish your retaining needs by making an access plate. A super-simplification of the access plate is to put a threaded hole really close to where you want to retain things and then use the head of a screw (or a washer) to hold the parts together.