As a calligraphy beginner, what features should I look for in an ink?
All the liquid calligraphy inks I know of are ready to use, no dilution required.
Pigmented ink must be well mixed before use for the pigment to be uniformly dispersed.
I find acrylic ink sometimes hard to remove from the nibs if not immediately cleaned
Iron Gall ink flows well and is a pleasure to use but requires an acid free paper. The ink will look very clear at first but will darken when drying.
India Ink (what's called 'encre de chine' in French) is probably the easiest to find and to use. A good one size fits all.
Chinese calligraphy sometimes uses solid ink stock which is diluted in water on an ink stone. It's OK for brush painting but not easily usable in pens and nibs.
For pens the main factor is what can you put in them. If it's a cartridge system you're more or less limited to what is available in those. (I have seen refillable cartridges before, but they looked like a mess to fill.)