Capturing that reaction is awesome
That fact that your making a big step in your life is phenomenal news, but the fact your making it a surprise is even cooler. Photographing that reaction is a great way to see how it all unfolded. Although you should keep in mind a couple of things before you set this up.
To keep this secret you’ll have to discuss the covert operation with the photographer. Where do you plan to propose? What time? What do you look like if you don’t get a chance to meet? The photographer can obviously pass off as a spectator or tourist of the environment and pop out when needed, but if you don’t discuss initial plan of the whole thing, the timing and expectation might be off.
Audience or no audience
You know who you are as people. Extrovert or introvert. The moment can be all to yourselves with no awkwardness from people looking at you. The other side is you want that attention as your version of telling the world you love this person. If so, you do have to maneuver around people to make this happen, along with the photographer who has to be there ahead of time and be in position to get the moment.
Time of day
Usually this wouldn’t matter, but you’d be surprised how this can affect your proposal. You set this up after your loved one is coming home from work. Traffic could kill the mood, not only because your waiting, but your future fiance could be in a world of road rage and not making the situation as romantic as expected.
The sunset idea is great, but from a photographer's perspective, we need enough light to work with to document the moment as it happens. That golden hour comes and goes very quickly. We can compensate with a flash, but the romance of the proposal may be interrupted by the burst of light thrown at your loved ones face.
So communicate and plan accordingly. You can come out with a beautiful announcement of your engagement for friends and family to see. Knowing what your photographer’s thoughts are for your vision is a good way to make this a time to remember.