I want to buy a heat gun, but I don't know what kind to buy (what temperature).
I want to use it for desoldering ssop chips (and not only), something like here for example: http://www.youtube.com/watch?v=AxYhF6Ab2CU

So what temperature should the heat gun be ? (min / max ?)

 

Well the minium should be around 500degF. Most lead free solders melt around 490degF

I use a harbor freight heat gun that I picked up for $19. It has 2 modes 572F and 1112F.

Also, pick up the kit:
http://www.harborfreight.com/9-piece-heat-gun-accessory-kit-95301.html

Using the attachment to bottleneck down the air stream a good 6 inches of so, and usind a directional shield on the end to keep the heat a little more than "generally" where you want it, really helps.

The 572deg F mode using an 8" black pipe tube (3/8"id) is great for this task. Connect it to the step-down attachment and drill a few holes to pull outside air into the tube and to keep decent airflow through the gun, works really well.

For through hole, I use it in low with no attachments and figure"8" around the area of the components to pull. This is a pre-heat method. When I am ready to pull After about 4 or five seconds, I stay about 4 inches away from the board and stay over the IC only moving about 1 inch back and forth while gently pulling on the IC.

 

Wikipedia has a pretty good article on solder.

http://en.wikipedia.org/wiki/Solder

When I was a lead tech on a line for Rockwell it used to erk me watching the assemblers hold a PCB under a "harp" (a dual output heading arrangement, top and bottom). Problem was, their heads would be swiveled talking to their neighbor which the board (PCB) blew a nice crispy bubble, which was delamination, destroying the unit totally.