Typically you need to replace the windows, the interior and exterior trim (casing), and expect there to be some sheetrock and/or siding repair to be done. But you also have to flash and caulk the window properly. Andersen (not Anderson) windows are vinyl clad on the outside, but wood not he inside that needs to be painted. Painting is probably going to be an additional cost.
You can do cheapo “replacement windows” that are all vinyl, w/ no casing, but they don’t look as nice.