Will the ring make it official? If not, why not give her a zoom ring so she can look forward to the honeymoon?
How to propose? My parents were married on Valentineʻs Day (1951). Why not, a) buy the ring; b) take it to the camera store in advance and ask the manager to put it in the display next to similarly priced optics; c) take her to the camera store on Valentineʻs Day asking for help to pick something out and surprise her with the ring.
Or else, offer her a broken film camera with the ring hidden in the film back. Offer it at a romantic dinner, and sheʻll be so disappointed at first, but then at the "right" moment, let her discover the true gift. If you do either of these, you must name your first child either Alex or Pentax.....love it or leave it.