I’m not the most expert in ski crossing but maybe I can help you a bit.
Here you can have an idea how the thing are done by professional:
1) Where we can rent the equipment like tent, pulka, gps phone, etc?
check on google or to Mountainguides.is asking for Hlynur.
2) Where we can finish? Hella or Grafarkirkja
Look that the winter is late this year and pretty unpredictable, we lack snow. On the newspaper today there is a pics of a place in the Highlands (kerlingafjoll) without snow. I personally think that if you can reach Hrauneyar on the way to Hella you can be pretty happy, southern than that is a lottery. If you refer as Grafarkirkja on the way to Kirkjubaerklaustur I think the crossing gets way harder and longer but probably still doable.
3) How was the weater in February-March
I think March better than Feb, longer daylights and bit more stable weather. April can be late and lack of snow. They say that if there is no snow for Christmas, snow will come for Easter. Maybe conditions will get better.
4) There are any huts on the way?
Few, Nyidalur and Laugafell. Check how to get access.
ww.fi.is and http://www.ffa.is
5) What do you think about the equipment, cross-country skiing or traverse skiing.
Check the pictures in link
6) We think to spend about 15 days, it is enough?
The crossing is doable in 10, you can add as many as you like doing variations, and also be ready for a couple of stormy day.
Enjoy and have fun,