If I were to purchase such a combo, it would be for no more than $40.
Opening the top would involve removing a couple of screws and actually taking the top plate of the machine off, and then looking inside at the condition of the gears and such. Looks like there is one on the left between the back thread guide and that silver knob, and maybe one behind the bobbin winder stem.
What do you mean when you say it only does embroidery and buttonholes? Looks like a standard straight stitch/zig zag machine with some extra built in stitches.