What should I expect in vet bills if I was to adopt a cat?

My sister is getting very weird about getting me a cat. I live with her son but she is convinced I am lonely and need a cat.

I know there isn’t a good answer for this since it will vary by cat.

My main concern is getting a crazy vet bill. How often does that happen?

And I also live by one of the busiest streets in Portland and intend to keep the cat indoors. Is one gender of cat better suited for being indoors only?