My big, professional quality machine that I use to make very high quality garments has two stitches: Straight and Zig Zag. That's it. And I can make magic with it. Okay, not magic, but I can make everything I want to wear with it -even t-shirts! It's not how many fancy built in stitches a machine has: it's how they are used that makes a finished piece high quality. Also important is the other things you do between sewing seams: cutting, pressing and finishing. High quality pieces all have well-finished insides so neatening seam allowances is essential. A row of zig zag stitches along a raw edge is sufficient, or using you can use bias binding which is attached with straight stitches. One of your 15 stitches is probablyt a really nice overcast stitch -and it will also neated a seam aloowance so it never frays.
Other stuff you need more than 100 built in stitches (85 of which are seldom used) I hope you have high quality scissors; they make a huge difference. A neatly cut piece of fabric is easier to sew than one that's been chewed up with dull scissors. Marking is important: carefully transferring pattern markings to the fabric will take the guesswork out of construction and allow for precisely matched seams, and symmetrical necklines, collars and armholes. Many pros use simple chalk or thread marking with cheap thread and a hand sewing needle. And just as important as a working machine is a good working steam iron. Pressing is just as important as sewing so get yourself a good quality iron with multiple heat and steam settings. Well pressed is well made -according to one of my long ago teachers. Also you need pressing cloths to protect fine fabrics from direct heat and a sleeve roll to press tubes -such as sleeves and pant legs.
When you get your machine out of the attic take it to a sewing machine shop to get it cleaned, oiled and tuned up. Sewing machines don't like attics; it's dusty, dry, damp, cold or too hot. And they don't like being poked and prodded in certain places -oil the wrong part and your machine will drip grease. Or worse, you could take it apart to clean it and end up with three extra screws and a leftover do-hickey and a machine that won't work. Let a pro clean and oil it. When you set up and before you sew anything get some scrap cloth, thread, and the sewing machine manual and practice basic operations: threading, winding the bobbin, straight and curved seams, changing needles, changing the stitches, which settings work best for each stitch, changing presser feet and more. Good sewers are completely familiar and comfortable with their machines and the more at ease you are at the machine the less you think about it, freeing your brain cells up for the actual construction of a garment. You can't think about how to set in a sleeve and match all those notches and match points -notwhen you are worried about your machine malfunctioning or the bobbin flying out and a needle breaking!
So your machine is probably more than adequate for fine sewing and those 15 stitches will make everything from a simple dress to a t-shirt to suit of coat. High quality is more about what you do with the machine and the little details than about the actual stitches used to sew.