I have a 101-4 in a model 40 cabinet allotted in 1931. Sweet sewing machine. Perfect stitches, no reverse, feed dogs do not drop and they take the class 66 bobbins metal bobbins and standard 15X1 needles. I had to have mine re-wired by the OSMG because the wiring was bad and they had been cut to remove the machine from the cabinet at one time. A manual is a must because it gives very detailed instructions on lubricating the motor and bevel gears under the machine. The manual is available for purchase online, That is where I obtained mine. I just purchased an generic feed dog cover for mine and put it on yesterday but have not tested it yet. It would be a plus if the one you are looking at has attachments.
I have a thread on this board regarding my 101-4 that I posted in early May.