Singer brand needles are not the best. I like Organ or Orange brands and I buy them in bulk on that auction site.
Choose the needle size by the thread size. For piecing cottons with 50 wt thread, an 11 or 12 normal sharp will be good.
The 15-91 was my main machine for piecing and quilting - actually, all sewing - for about ten years. Hope you'll like it just as much as I did.
