Building staircase from first floor to the attic with a Dorm window for headroom. Large 40 x 12 attic space.
You really need to bring in an expert to evaluate the building structure to see if what you have in mind if even feasible. There is little chance that your short text about your intentions can provide the detail needed to determine feasibility. Also since the area is probably enclosed from below at the present time pictures would likely not help too much here either.

Some things to consider:

Adding a 5 or 6 foot wide dormer opening into an existing sloped roof will likely require some major re-work of the roof structure in that area. 

Many houses with attics have minimally sized ceiling joists that may not come close to supporting a floor from above. It is likely that some new joists between supporting walls will be needed to make the space safe but that could reduce your existing headroom to too much.

There are a lot of old style constructions that had center support posts under the rafters in the attic. If  your space is like that you may not be able to legitimately remove them.