300s arn't that rare. We have two of them and several of the other guys on this forum have them and 350s. Both the 300 and 350 were rowcrop tractors available with bar axles and narrow or adjustable wide fronts. They were also available as utility tractors with flanged rear axles and HD cast I beam front axles. The 350 was a bigger version of the 300 with a 164cu.in motor and 36" rears in the row crop version. Motor and tranny parts are common to a large group of Case tractors. Some body parts can be a bit spendy. Roger Hornbacker repairs the gauge clusters, and price varies.
The pics is of the 300 I use all the time. The other 300 we use on the farm during the summer.