Hi,
Sounds like your describing a Wheeler and Wilson model D-9, a fantastic sewing machine. Bobbins and the slide plates are frequently missing and are in great demand, so prices are high. I'd search ebay, past and present, to get an idea of expected cost. Value is personal and regional but I would not pay it with missing plate and bobbins. Dates to circa late 1890's. Uses obsolete 127x1 needle or MY1014 industrial can be used as a new replacement. Perhaps you can use the missing plate and bobbins as a bargaining tool. Good Luck.
Jon