replacing dormer windows on an old roof

hi guys,
i'm considering buing a hundred-year-old cottage (terrace) which has already had a loft conversion (2nd floor). The two back rooms on the 1st floor are compromised by the slope of the main roof. They have decent floor-area but you can't stand up in them; they each have separate, small dormers which offer a small amount of standing space in each room. My idea to make those rooms more useable is to replace the two small dormers with one large dormer which encompasses them both, but I have no sensible idea of the magnitude of such a job. Are there any pitfalls I should be aware of, and how can I find out a very ballpark estimate of the budget required for this job?

thanks.