What's the bird situation there?
I don't expect dumbo guppies are going to be well camouflaged for any predators. My urban, six months of the year pond used to get visited on a regular basis, and even with lots of cover, and with naturally shaped fish, some fish got picked off. Some carefully chosen, broad leafed plants on the edge would help.
A winter that gets to 4c won't work for year round fancy guppies. Pond heaters are a whole different game from aquarium heaters, that will burn out at those temps. They will also be very expensive to run. I would see it as a summer project, with the fish coming in when it gets to 12 degrees at night. With a Spring, summer and early Fall of natural foods and indirect sunlight, the fish will have great colour when you bring them in.