It IS possible to buy a solid wood exterior door, but I agree, if its below grade, you're better off with a steel - or better yet fiberglass door. You can't easily cut either of those, but you can custom order one to size. If you want to go with a wood door, you can make it pretty much weather impervious, but it will require 10-12 coats of valspar varnish / urethane, and by the time you're done, you'll wish you got the fiberglass door

I have a wood with glass inlay exterior door as my front door. Its wood and glass - no fiberglass or steel involved. And yes, it has 12 coats of valspar urethane on it.