I currently own a 1917 66-1 Red Eye Treadle. A very popular model! Easy to thread and it sews like a dream! Makes a perfect stitch. It uses back mount presser feet and attachments. Of course with a treadle there is no reverse or zig zag capability, but nevertheless, a fine machine to sew with.
The Singer 66-8 is an electric model, with reverse, but of course is straight stitch only. This Model 66 is a heavy duty machine & has the capability to sew through various types of fabrics. It uses low shank presser feet and attachments. Again has the drop in bobbin which is a favorite feature of mine. This is a simple easy to use machine. These machines are also very easy to work on, and with a little care can continue to provide many years of faithful service. What a wonderful piece of history to pass on!
Singer came out with the Model 99, a smaller version of the 66. Then the 99 was updated & turned into the 185K.
I highly recommend a Singer 66. Parts for this model are readily available. With other versions of the Class 66 available, you can choose the one that’s right for you.