I was hoping that someone with more knowledge than I would write first.... it is my opinion that any tractor is worth purchasing if the price is right.( I bet someone will take issue with that comment!)
 Most any of the hydraulic pumps can be rebuilt. Do you know if it is internal or external? .... probably external.
   If the oil pressure is low, it may be that the oil pump needs to be replaced or rebuilt which is no big job or it may be that the oil is old and the filter and oil need changing if it has been sitting a long time. a good flush of the engine sometimes helps. I would do these little things before pulling the oil pump. ( how low is low oil pressure?)
    As to the serial number, there is a number on the engine too, which will give you a date of manufacture but I don't know where to tell you to look for it. I am sure there are many more knowledgable than I and they can help you out.
...........Bottom line is how much are you willing to do on the machine and how much is it worth to you? Only you can decide if you paid too much.
                                      Cal