I'm having all the windows in my brick house replaced with vinyl high-efficiency, laminated, hurricane-proof, low-E, argon-filled, high-octane, low-fat windows. It took an experienced crew 3 hours to do the first window before retiring to think it over a bit. My advice based on this experience would be to proceed as you're doing, and try one window before investing in a lot of them. I don't have any experience with sash-only kits, so can't advise you there, but a quick Google search found several satisifed DIYers. I don't see how that would work in my case, since the old windows are all aluminum single-hung windows. If you decide to try a complete window replacement, tear out the old window first, to see exactly what kind of hole you'll have to put the new window into. We ran into some weird construction these guys hadn't anticipated that threw their measurements off. What kind of windows are your originals?