Replace or Retrofit Windows?
It depends of a few things.
It depends on whether or not you can find a straight replacement.
It depends on what you have outside for siding because that has to be finished off after the windows are put in.
It depends on wether or not you can find the 'style' you want in a retro or new window.
Talk to your contractor about all these. Different mfgrs. do different things so you'll find that whatever you end up doing wasn't easy! That's just the nature of the beast.
Be sure that the Contractor re-insulates around the frame, and that the price includes the windows, the installation, the finish work inside, and the finish work outside (including painting if necessary.)
Now, about that mold/mildew (M/M). Do you know what caused it? M/M on the OUTSIDE of the window is a clear indication that there's excess moisture out there, a relatively high temperature, and low (or non-existant) air flow along w/little (or no) sun. It sounds like you need to do some pruning to improve air flow (to dry up the moisture AND allow some sun in there!) That 'stuff' isn't healthy and breeds all other kinds of small problems. If you want more help w/this, give me more info (like what the landscaping around the window area is like, including tree and sun 'locations' on your lot.
My best to ya and hope this helps.
Jay J -Moderator