It would be a good idea to take the machine in for servicing to make sure that all of the lubrication is still good and in place, and that everything on it still works as it should. Sitting unused for a long period of time can create problems in a machine.
My uneducated guess, based on what info I was able to scrounge from some internet searches, is that a good price for this machine would be around $300 with a range of about $200 to $400 tops. A lot of the older computerized machines are not valued highly because it can be expensive (or impossible) to replace a motherboard if it goes out.