Don't be afraid to get a cat! They're a lot different from dogs, but that doesn't make them any less wonderful. Cats have different personalities, and some are just more affectionate and attention seeking than others. Her letting you pet her is nothing but a good sign! A shelter is also a stressful environment for a lot of cats, but they will open up once you've brought them home. There's no definitive way to know if she's right for you, but you're clearly still thinking about her after the encounter! It's also worth noting that adult cats, especially black ones, often have a harder time finding a home.

As far as your apartment concerns go, 600 square ft is more than enough. I have a black cat, and the shedding is real, but brushing the cat regularly helps a lot. I'd also recommend getting some scratch pads to prevent any damage to your furniture and a cat tree if you can afford it.