Need replacement window, with lintel

How would a window fitter or builder approach adding a lintel to a window that doesn't seam to have one, and replace the window. So I know when getting quotes if their approach is right. The window is old and bowed outwards, and upon removing some of the plaster above the window, I found the brick on the outside is only held up by the window. Is there also rough estimates on costs? What would i expect to pay for a new lintel to be put in above a large window, and the window replaced. (2.5m x 1.2m in size, casement window).