The grinder upgrade I put off for two years
For two years I used a cheap blade grinder and told myself it was fine for cold brew, since the grind is coarse and the steep is long anyway.
It was not fine. A blade grinder produces everything from dust to large chunks. The dust over-extracts and turns bitter, the chunks barely extract at all, and the dust is also what clogs your filter.
I finally bought an entry-level burr grinder. The first batch was noticeably sweeter, and filtering took half the time because there was so little fine powder.
If you are going to spend money on one thing for cold brew, I would put the grinder ahead of fancy jars or special filters. Or buy pre-ground coffee for cold brew from a roaster you trust, which is better than a blade grinder too.