We have a spiral staircase up to a finished attic-like space in our weird old house (that we rent). In general, its fine. But it is a very narrow spiral. That makes getting things up and down challenging. We only use the space for storage so it is fine, but if we wanted any furniture op there, the only option would be to carry up Ikea flat packs and assemble up there, and leave there for ever. There is no way we could get even a twin sized mattress up there, so a bed would be out ((other than a futon). So if you want this to be a potential rental, that's something to consider. With a wider spiral, it might not be a problem, and there is no permanent rail at the top of ours, which does make it surely not up to code, but there's a small bookcase there that blocks it off so no one would fall, but can be removed. It cuts the amount of spiraling we have to do to get things up there. So a removable rail, if still safe and up to code, might help. But really just making the radius of the spiral as big as possible is important. I'm too lazy to go up there to look because I'm tucked under a warm blanket at the moment, but I'd estimate our steps are perhaps ~~~15"wide.
That makes it sound very challenging, but really, for storage it works quite well and getting storage bins in and out on the spiral staircase has been fine. It is certainly nicer, IMO, than a pull down stair case would have been.