For that usage, you're probably okay with a mirror lens. I tried a Nikon 500mm/8.0 once years ago, and among mirror lenses, it's pretty sharp and if the mirror hasn't oxidized, it's probably closer to f:8 in light transmission than many cheaper mirror lenses. With architectural subjects you don't really need the speed of a refractive lens (as you would for wildlife), and you are likely to be far enough away from the details you are photographing that you won't have any out-of-focus donuts or double lines.

A sturdy tripod, solid head, cable release, and MLU are recommended.