After a fashion, you can. The DreamHost Wiki shows you how (linked from the "documentation" link in the Control Panel page for announcement list. Baically, you are just "copying" the list of members as text more processing to import somewhere else
Not with the announcement lists, though it is pretty trivial to archive the announcements on your your site and make them searchable there.
None (also linked from the control panel page for announcement lists). Just make sure that you follow the Anti-spam policy carefully. You won't have to worry about that as long as you only mail to those that sign-up via the list itself, but you should be very careful if adding members in any other manner.