To try to keep with the character of our 150yr old home, I am
considering buying a real wood (pine) frame storm/screen door rather
than using the IMHO cheapie-looking aluminum or vinyl doors that just
bolt over the brick molding.

First, other than higher initial cost and the need for maintenance,
are there any other negatives to wood frame?

Second, I would like to stain the door to show off the wood and be
consistent with the stained wood main door.

Couple of questions:
- What stain should I use?
Is a regular Minwax stain (and sealer since pine) good enough for
such exterior use?

- What (clear) finish is most appropriate for protecting the door from
UV and other weather damage? (we live in the Northeast)
Would you use a spar urethane?
How many coats would you ud

Note the door manufacturer (The Combination Door Company) "recommends
the use of Sikkens Cetol Door and Window finish." Are such finishes
anything special or are they just pitching somebody elses product