Here's a page with some info about serial numbers <link>

My suggestion would be to look for something circa 1980 that appears lightly used. My rule of thumb is "If a boomer received it as a wedding present and never used it, it's probably worth buying." That seems to be the sweet spot for these sorts of appliances--lots of high quality items were being produced and sold, so they're easy to find in good condition, and relatively easy to find parts for.

With appliances in general I look for indications that it is user serviceable (housings held together with screws are a good sign). This isn't a guarantee of quality, but it's a good start. "Good" depends on what you want to use it for. Even a mint condition kitchenaid from the 70's might not stand up to daily use making stiff bread dough.