The door has to determine what it needs. How far you have to go depends on what finish you want to end up with, and how deteriorated the wood has become. To attain a perfect new-looking varnish, you would need to sand down until all greyed weathering is gone, and all surface checking is eliminated. OTOH, if you'd be happy with a mid- to dark stain job, then the weathering becomes less of an issue because the stain will cover a lot of the damage. To simply paint would be yet less prep work.
We don't know how bad the wood is without a picture.
Casey

First off you will need to take it down it will be dam near immpossible to refinish it while it is hung in the frame. As far as stripper that would deped on how bad the finish is and what your looking to accomplish with the new finish. I would suggest going to a quality paint store ans speaking with them as far as products to use.