Before even considering this proposal please contact the local planning department and check the listing status of your barn. If it is listed, conservation officers are often reluctant to allow any new or adapted openings to those present within the historic structure. If you do get planning consent, your plan sounds considerably more ambitious than simply replacing some windows. It may be worth consulting an architect who will know the best specialist suppliers. Also sound out a few local bespoke joinery manufacturers for ideas, and if you have seen a similar new design in a local selfbuild or conversion, why not write to the owners about their experience?