In thrift store shopping you often have to go on your gut feeling, and if it feels right I go ahead without any research.
Condition is key, you can find machines that have been sitting unused in the corner of the parlor for 100 years, or out in the barn for 100 years, or used for 100 years too. I'd say that for me, I'd be looking for perfection in the decals and the surface, that the cabinet is also in good shape, etc.
I'm cheap and I live in an area with a lot of high income people and my thrift stores are awesome and I often see some lovely machines in cabinets and regrettably can't take them all home. For me the price would be a bit high unless it made me all giddy with excitement and jump up and down!