I'm late to the game, but thought I'd add my 2 cents-
1) I doubt very much that using unfiltered water will work. If you don't want to spend the money on a RO/DI unit currently (there are used/refurbished systems available), then store bought RO/DI water would seem to be a decent temporary measure. Unfortunaetly, in the long run this would cost more, but less so in the short term.
2) If you keep the canister very clean, I would see it as a once in a while measure for cleaning particulates out of the water (unrinsed sand being one of them). Alternately, you can use it as a chamber for carbon. I don't know if a canister would work for GFO or biopellets if you ever went that route.
3) Any refugium would be helpful, the larger the better. As to the 3 gallno vs the HOB, that depends on your budget and willingness to plumb stuff.
4) Agree with adding live rock. Options would be dry rock seeded with a piece of live rock, aquacultured (Tampa Bay Saltwater or Gulf View for stuff that literally sits in plots underwater off of Florida, Dr. Mac and others for rock that simply sits in huge tanks at a dealer's to develop bacteria), or wild (my personal bias would be to not do this to try to preserve natural reefs).
5) Agree with testing to monitor your cycle - check ammonia and nitrites. When they drop to undetectable, you can start adding stuff slowly.
6) Strongly agree with reading, reading, reading! There's a lot of info out there.
7) For a "not really a refugium," you could simply have some macroalgae in the tank as a way of keeping nitrates/phosphates down and prune it (tear bits off as it gets too big). Don't get Caulerpa (my bias, again) - it wrecked my last tank.
8) You can use a HOB skimmer in place of a HOB refugium, and then when you get the sump run the HOB skimmer on that. I'm looking to try something similar with a Reef Octopus BH-90 using that method (Lifereef overflow).
9) I try to keep my setups simple, so as an example my tank would have Tampa Bay live rock over (relatively) fine sand, a HOB overflow, a few powerheads, and a refugium and return pump in the sump. A HOB skimmer in the sump if I need it, and macroalgae in the refugium and display. If I'm feeling really simplistic, ditch half of that and go with live rock + sand, macroalgae in the tank, a few powerheads, and keep the fish load low and enjoy.
Good luck!
