Kostas. I would really recomend you have a place like www.cheapbatterypaks.com do the job.
They do what ever you want to please you and if they screw up, they eat the loss not you.

Look at the sites selections and specs. of the cells used and the $$

I am dam good at soldering. But I screw up a little once in awhile if the tabs are not tinable.

If I was next to you . NO SWEAT.