There are White rotary enthusiasts. I think Treadleon is a group devoted to them (they often convert regular White rotaries to treadles because they are such workhorses). Check groups.yahoo.com for a vintage White group, and also Google Treadleon for more info (seems to me it was Captain Jack who headed that group).
All that being said, I personally would not buy the machine as it's really heavy and there is very little (if any) resale value if you ever get tired of it. I also think $75 is too much. These machines often do not sell well even in thrift shops because their weight makes them very difficult to move around. I would recommend caution. Do some research online before making a decision.
I don't think the serial # on a White makes it easier to date, as I don't think records were kept on the serial #'s. It's mostly Singers for which serial # can be useful for dating.