I want to sleep with my work friend
Does she want to sleep with you?
I want to ask her, but I don't know if she's seeing someone
You can't help who you are attracted to but sleeping with a coworker is not good. If things don't work out with them it will be an awful place to work.
She's so prim and proper. I want to smash it.
I think it would be best to just let this remain an idle fantasy and then move on.  It sounds like you are very different in terms of personality, so a long-term relationship probably wouldn't work out.  If you sleep with her one time, the workplace will just be awkward thereafter.