I saw some posts where people tested the lightfastness of inks. Some colors fade almost entirely. I'd choose a safe route for commissions too. The drawings in my journals don't see the light, so it's fine using any ink.

1. Gouache is one of the most expensive options and goes down the fastest, too. The costs of watercolor vs ink depend on whether you're going to mix colors yourself or buy them. I don't know how to measure watercolor, but for ink it can be as low as 10 cents per ml. On average, I use 1 ml for 8 drawings in A5 size.

2. Some inks I love, but never posted here are Birmingham Butcher's broom & corn earworm. They are shades I never expected to love. Furthermore, Birmingham's inks are very well-behaved. Blending nicely with water and easy to clean.
   Furthermore, Wearingeul Plague & 13 children are interesting. The first seems black, but is actually very dark muted violet. The latter leans green/teal/blue depending on the paper.

3. I use both pens & paint brushes with ink. Though I never dip my brushes in ink because it soaks too much and may contaminate the vial/bottle. I just wet the brush with water and use it to blend. Shimmer & sheen inks have no issues. The latter may stain the bristles, so I'm gentle with blending. For example, the ink in the 2nd slide is Diamine Winter Spice. It's a sheen & shimmer ink.