IME unless you like fish that are absolutely not reef safe (triggers, groupers, etc.) you will end up going reef. You will fill up the tank with fish in a few months, get bored because you can't add anything else, and the frags at the LFS
will be too tempting. You will start with mushrooms and zoos, they will do well because they always do unless you take them out and take a blow torch to them, and you will be addicted. From then you will make upgrades to favor your corals more and more and then you are stuck with a $200 useless filter because it isn't ideal for your setup.
A sump will fit in your stand. If you do it right and plan well a 30long will fit in the stand and hold a skimmer and a refugium. The refugium will do its job if it lowers the nitrate and phosphate concentrations at all. You know this is happening if the macroalgae is growing at all. It will take up no more room than the tank will take up anyway and will improve water quality.
The FX5 is a great, sometimes unbelievable, filter. But like any canister it was designed for freshwater use. It has a lot of mechanical media and room for A LOT of biomedia. The biomedia will be useless since you will have live rock and the mechanical need will depend on the tank. I had no mechanical media in my tank at all. I wanted to keep the debris moving until the filter feeders ate it. I did not want to trap in one place so it can rot just (releasing nutrients that will feed algae) and starve my filter feeders and require me to feed more (putting even more nutrients into the system).
I am not trying to talk you into anything. You will do what you are most comfortable with. I just want you to be as informed as possible before you make your decision so that you really understand what your tank needs and how to meet those needs.