I have a 2015 1.25 studio fiesta 

I love the car and want to start modifying it the main modification i want to do is sawp the drum brakes for discs can anyone recommend the best products and method of doing this 

Fit the complete rear beam from an ST.

But is there any point? Also Ford designed your car to have a certain balance between front and rear braking force when you press the pedal. I would imagine putting rear discs on it would alter that in favour of the rear brakes, which is not a good idea.

There was topic on here a week ago about changing to disk and said it was not worth the hassle and then the people who have done it said it’s a small noticeable difference, if you can get the set up cheap then go for it, IMO as it’s the 1.2 I really can’t see the point in it apart from looks 👀 if your thinking about going on track with it then just upgrade the front, ebc do a great disk and pad upgrade  🤷‍♂️

Disks do look better than drums and an important part of the mods world is looks.  So why not, it would be a great learning experience if nothing else, and it will make the car look like a better spec version.  You'd hope, that as higher specs do have disks, you could swap in the whole system.