Water has rotted at least the window sill and probably deeper. I'm looking for ideas to eliminate this problem, short of removing the window and having to deal with the symmetry problems arising on the exterior. I'd be interested in having help with this; I'm unable to do a decent job. Can you suggest the next step or two?
Generally, we advise people (who want to keep a window in that location), to: Order, and install a vinyl window. Order it without window jambs. When installing it in the old window opening, rather than using wood trim work, use PVC composite (non wood) trim. Use it for the window jambs, casings, and apron. When installing the stool (interior sill), pitch it slightly for water drainage. Use mold/mildew resistant caulking. Paint with an exterior grade, mold/mildew resistant paint. If these things are beyond your skill level, consider highering a qualified and experienced carpenter.