Mike Hay, info helps. What I can tell you is what you "probably" have in store. What you still need to do, tho, is get out that phone book and have a structural engineer size things for you. Local codes and soil conditions, to name a few of the variables, still haunt you and an engineer, your new "buddy" will solve them for you.
Fifteen feet is not a big problem to solve. A single story home with a roof load, even if it is concentrated on the wall, can be spanned with a steel beam. If you are going out to the perimeter, your engineer will tell you if the concentrated loads created when you install columns under the beam can be supported by your existing footings. If not, you will have to install new footings under the columns. Not a big deal. If you have a slab on grade condition,you have to saw cut the slab, probably 30" square by 12" deep, add reinforcement bar per your engineer, put in your base plates and install you column and beams, and you are basically done. Of course, you will have to temporarily support all that is being held up by the wall. Your engineer will give you all the specs.
You need to make sure this is ok with your city building inspection dept, and meet all local codes.
There, that is what you have to look forward to. Don't take chances. This is not a big job and if you have it hired out, which we recommend, you shouldn't be out too much. If you "can" handle the work except for the structural specs, then I say go for it. With another hand to help, you should not have too much trouble.