Yep that would work, the other option is a heated water bucket

I was thinking about keeping the stable at 5 degrees celsius so the water doesn't freeze and putting a warm winter blanket in the day. We also get very cold weather where I am from.

We try to stop the stables from going below -15 though where I ride. (I just know many other stables in my area where it gets colder and the horses are just fine).

It really depends on what the horses are used to...
I live in a colder climate, and many people take horses in at night and leave them out all day. Whenever it's below around 5 degrees Celsius we blanket them all (when it's warmer only some of them need it).
Stables around here can go down to -30celcius and the horses are fine. (We do heat them but it doesn't make a huge difference)