What You Actually Need
For costume construction, the most important machine capabilities are: a reliable straight stitch, a usable zigzag stitch, the ability to sew through multiple layers of medium-weight fabric, a walking foot or built-in walking foot function for stretch fabrics, and a buttonhole function if you make garments with buttons. Everything else is a nice-to-have.
Recommended Starting Level
The sweet spot for serious beginner-to-intermediate cosplay is a mid-range mechanical machine from a quality brand — Brother, Janome, or Bernette at the $200–$400 price point. These machines are reliable, have the features you actually need, and will last through years of project work. Avoid the cheapest machines (they lack the build quality for sustained use) and don't buy a heavily computerised machine until you know you'll use the features.
What to Skip
Embroidery functions are unnecessary unless you specifically do embroidery. Hundreds of built-in stitches sounds impressive but you'll use four. A large extension table is genuinely useful. A decent selection of included presser feet is genuinely useful (especially: a zipper foot, a buttonhole foot, and a walking foot).