I'm confused, at the start you say brackish would be neat but you go on to talk about FW
and stock such as guppies. If brackish is your aim, look in to mollies if you want to keep with closer to FW
fish, as they like a little bit of salt. But the rest of my advice will be given as if you're going for FW
I also don't understand where abouts in a room this'll be placed. Where is your coffee table currently? Is it in the middle of a room or corner or side? Will a tank be easily accessible for maintenance?
Do you plan on it being a riparium kind of setup? I'm not sure about your lighting but just be sure you get something strong enough.
As far as filtration goes, if youre set on something underneath the substrate, I'd try and find whether it's viable to run an under gravel filter, that will suck the water down through the gravel then try hook it up to a canister filter so that filtration can occur underneath the tank. Then from there you can run it back to your spout at the mangroves or wherever. Only issue I see with that is detritus build up in the gravel.
An alternative to that is running a sump underneath. There's a number of benefits of using a sump, if you go down that road I'd read up on it a lot before hand, and get the tank drilled for overflow, as opposed to a HOB
style overflow. There's no NEED for a refugium, no. However if you go with sump filtration, you have a very large footprint to work with under the tank, so I personally would seriously consider adding a "fuge" section to my sump. It can only be beneficial, really.
With evaporation, you could look at an automatic top off system (ATO). I don't have any experience with it but I'm sure someone does. That, or you could have a container of a sort close by the tank (I'd put it under, as again, you have a large footprint to work with) of pre-made water (dechlorinated, with desired salt added if you go brackish) and figure someway of making it convenient to get back into the tank (a small submersible pump connected to hose pipes perhaps). Additionally a sump will help control your water level, but you'll have to monitor the water level in your sump so it doesn't run dry.