An outside door, if you go with wood, go mahogany(SOLID). Outdoor furniture, cypress and doug fir are cheaper alternatives)
For window frames , Id suggest hard maple (Takes paint nicely) and Id really have it clad in some kind of acrylic facing out.
I redid a rowboat with cypress seats about 10 years ago an we beat the **** out of it and it sits out all winter. The cypress , I just power washed it, scrubbed it, brought it into a barn to dry, then resanded and polyd it. It looks like new.
I love cypress, its a water grown wood so its learned to del with moisture.
Its amazingly stable . Only thing, it has a boring grain, its like a very stable poplar.
NO oak, itll rot in 2 years if you live in a temperate area with 45" of annual rain or more.
Anyway, even if your present sills are rotten you can make wood stabilizers . (I made one out of epoxy glue and xylene that works just as good as any commercial wood hardener)