Aloha Andrew,
Sounds awesome. I've done dozens of these. I have just the right rock for these tanks. I also have spare 10 gal tanks and specialty tanks. I have a 4 gal hex that would look great on a counter top. I wouldn't worry about an external filter. The tank will not need it and will look better without it. Coral and algae filter the water and water changes keep Alk, Ca etc. at good levels and problem free. I can't fathom any marine tank without coral. Sure, I have seen hundreds of tanks without coral, but they didn't know what they were missing both in filtration and beauty. Even predatory fish tanks can keep some types of coral. You know where I live. See ya again soon.