When it comes to the opening, you pick out a approved egress window and cut the rough opening to make sure the bootom of the open window is a low as the required height and not any higher. This maximum height limit is set to allow children to get out. I have heard of people having to get a code approval (whether required by law of just common sense for resale) before the carpeting was installed.
You should also make sure your exterior well is sized to permit a firefighter down and get ino the interior with gear on.
The lintel size and type will be determined by the material above the rough opening and the load on the wall, even if not a loadberaing wall, it does cary the dead loads of the walls above. An engineer should be used for the lintel design.
Usually it is better to use concrete sawing contractor since they can come in and cut and remove the debris in an hour or two since they have the equipment and knowledge.