While most cats would not normally bother larger chickens, should you ever have chicks (however you obtain them-buy or raise), they would be in danger because of their size. Another point: cats can and will climb fences, no matter how high, IF they really want to get to the other side. I've seen cats climb walls, literally! If there is any way they can find a foothold (or clawhold) and they are motivated enough, they WILL climb.
Also, just because you've never seen any predators, please don't assume they aren't or never will be there. The scent from the chickens alone can bring predators to your backyard. You may never have a problem, and I sincerely hope you don't, but I'd advise you to never let your guard down.
We have many predators here: hawks, owls, coyotes, raccoons, dogs, cats (I have my own cats and one dog). I don't have chickens (yet!), nor a coop, but I plan on making it as predator-proof as possible! I'd love to be able to let them free-range, but I know that would just be asking for trouble. The coop would have to be as secure as Fort Knox, and the run completely enclosed, with an apron around the entire perimeter (not buried, we have clay here and trying to dig to bury it would be extremely hard for me to do, since I am the only one who would be doing it). Though I've never seen snakes around here, I would make it as hard for them to get in as well, because I know that there are snakes around.
Good luck!