That's one of those things that varies per person, but based on what you described and if it were me probably
Summer kit with long sleeve light long sleeve lycra baselayer, leg warmers, short finger gloves, ultra light sugoi wind shell or maybe my showers pass double century rain jacket. I would also wear a cycling cap to keep the water from running down in my eyes, but I don't have one. I would also ride this metric century as quickly as possible and not stop unless I needed a nature break, but again each of us may have a different take on this.
I would also consider some shoe covers that are fairly water repellent. At this time of the year I usually have a mix of my winter and spring gear in the bag so I can pick out what I need just before the ride. So I typically have more available in the gear bag than what I listed above.
Edit: That is really close to a temperature that I will carry the jacket in my back pocket. I would rather get wet than burn up in a wind or rain shell if the temperatures get warm enough. But I would think that is going to be cool enough to keep a light jacket on.