I'm thinking of using the photos I take as a fetish/erotic photographer? NSFW
Use them how? Use them to make money?
If I make a good amount of income from this I could buy a new lens.
As long as your clients are aware and consent, I see no issue. Buying a new lens will expand your hobby, too.
Then do you think it's a great idea?
You are an adult and can make your own decisions, so yes. Your body and your choice, as long as all parties involved are okay with it. Sex work is a legitimate profession!