If I change a question in the question bank, will it be changed in all the quizzes it appears?
Yes. See Tim Hunt's forum post https://moodle.org/mod/forum/discuss.php?d=234011&parent=1016542#p1016558
Is there a way to set the default for essay questions on our site to allow uploading of one attachment?
The only way to change this at the moment is by editing the code.
In question/type/essay/edit_essay_form.php, find where it sets the default for 'attachments' to 0, and change that to 1.
Note that, after changing the data in the DB, you will need to go to admin -> plugins -> caching and purge the Question definitions cache. Or just purge all caches.