Many designers do great work without sewing, but I design mainly by sewing. Since I began working mainly with knit fabrics, in developing the Pixyworld line, I realized that my sweet little Huskystar 219 (Husqvarna) wasn't going to be adequate for sewing my samples.
I first acquired a serger, used, which was a good start but a bit of a disappointment. It arrived broken, and after the first repair I was able to use it for awhile. I then started having trouble with the threads breaking, and Leabu showed me that it was because of a design flaw, not a problem that could be repaired. My new serger, only about $500, is the Janome 1110DX, and it is working just fine for me.
I then splurged on the MemoryCraft 350E, an embroidery machine which I needed to embellish my designs. It was very inexpensive for an embroidery machine, and I have had no problems with it so far.
Finally, I got my CoverPro 900CP, on sale for $300. Nearly all my hems are coverstitched, and it does fine with those. It does not have a narrow gauge setting, which I would have liked, for foldover necklines, but I am happy enough.
If I were to choose on more machine, it would be a 5-thread machine for topstitching. But at present I can use covertitch, upside-down, to create an approximation for factory samples.