I think your idea to keep the original charm in the areas other than kit and bath sound good. However, for the kit, i would try still to keep the charm in the doorways and windows, upgrading the counter surfaces, backsplash, sink and appliances, maybe add an island if there's room. if you do new cabinets, i'd recommend making them a little 'old world', rustic or perhaps Tuscan, depending on what you see or hear from other realtors or home pictures.
I dont know too much about it, but the historical district recently had a tour of homes and perhaps there are some online pictures that you could take advantage of.
Have fun, & Good luck!
