I was wondering, when would it be suitable to start planting things like broad beans, corn, french beans, sunflowers and other large plants?

The maximum temperature in my area is only around 14c during the day.

Each plant has its own individual requirements. A good start would be to read the instructions on the seed packet. Better would be to get a good book on growing vegetables.

It is usually the minimum rather than the maximum temperature which affects planting time.