Front left and rear left door buttons and lock stoped working. I noticed that under the rubber some wires were broken or almost broken.

 

Is it possible or viable to open the door interior with instructions like this: http://www.instructables.com/id/%C5%A0KODA-Superb-MK2-door-panel-removal/?ALLSTEPS

 

and then pulling the cable so much inside that it could be repaired.

 

Planing to use welding  iron + heat shrink wrap on it and then putting it back.

 

The cable set where I asked was almost 150€, is it worth to try to fix it or better to replace the whole cable set?

Repairing these wires is what most people with this issue do. Unless it's still on warranty... 

Ok, sounds great, and will it be just that that the door needs to be opened to get enough to the cables?

I've done it with it all in situ. I removed the door strap to allow it to open wider.

My wife then assisted with holding the rubber boot back whilst I got in there with a soldering iron. It was fiddly but doable. Took about half hour from start to finish.