I'm not familiar with a BZ-15-8. I did a search and from the photos online, it appears to me to be a 99. On the serial number, you are missing the letters that precede the numbers.
That said, if you can get a 99 in good visual condition for less than $50.00, you are doing well. Generally speaking, if it's an old singer and it looks good, it is good. You may need to do some cleaning and lubing, but they are almost always great machines and come back to perfect working condition very quickly. Unless they have been stored in poor conditions, they don't give you any problems and will sew perfectly and make beautiful stitches.
BUT, another "that said" - they always look better in the auction photos than they do in person. If you get an opportunity to go to the preview, do so. If not, don't overbid. Without actually seeing it and making sure it turns, I wouldn't go over $40.00.
bkay