Good plan to take things slowly
It is a necessity with saltwater even though its soooo hard to do! I started with a 20g tank and ended up adding a 10g refugium underneath. Without having the foresight to have my 20g drilled before setting it up I added in a HOB
overflow. In a 20g tank it takes up a ton of space and took some modifying to get quiet enough to not annoy the crap out of me (wasn't difficult to modify at all) In a 75g a HOB
overflow wont be as prominent but you might end up wishing you had drilled it.
A refugium isnt necessary at all but is a big help with being able to add macro algae and culturing copepods if you want to keep a mandarin goby or other fish that lives off of copepods.
Also, building a refugium can be as simple or as complicated as you want. Most people add baffles into a refugium to cut down on micro bubbles. I skipped that part and haven't really seen and issues with the microbubbles yet