In fashion school, I spent so much money on various tools and materials from special marker paper, to special tracing paper, colored pencils, light-boxes, markers, watercolor, etc. The investment was somewhat justified in having the opportunity to experiment with media as a student to find which materials resonated most with me.
The honest truth is that the tools I use to design most often in a professional setting is a regular mechanical pencil and standard 8.5 x 11 paper.
Fashion design doesn’t have to involve a huge investment in materials. A simple pencil and any piece of paper will do. Your greatest asset is your creativity, some knowledge of garment construction, some knowledge of fabrics, and a pencil and paper. If you’re not the best artist—who cares. I know a lot of designer who aren’t the best at drawing but leverage other of the many skill sets involved in being a great designer. Plus, anyone can learn how to draw clothing with practice.
When it comes to your portfolio, rendering in a media of your choice is necessary to communicate color and fabric, but the backbone of your collection is the thousands of small decisions you make about silhouette, body length, neckline shape, etc. The rendering is a cherry on top.