Specific answers below, but here's a link to a great comprehensive quide on the Boxster Wiki - see option 7
does anyone know what the maximum mounting depth is for the dash speakers?
it varies by MY
I am planning an upgrade on the dash speakers and will be using the factory speaker basket.
good choice - easiest way to do it
Also, does anyone have any suggestions on which speakers to use?
pick your poison - I used Infinity Kappas and was very pleased
What about components? Is it possible to mount the tweeters? If so, what size?
Yes, very possible as there are (using the term VERY loosely) things that I guess are supposed to resemble components in there. Do you have door speakers?
All in all, upgrading the speakers is very worthwhile but do remember it's a cab and not the best shell for a premium sound stage. Besides, the best speakers in the Boxster's sound system are at the bottom middle of the rear bumper
Good luck 