Random thoughts and paranoia to follow....
I've found the GPH rating on the Eheim 2028s are a joke. They don't seem to push much. The 2080s are supposedly in the 70% range of rated flow but any clogging will drop that fast so you'll need to keep up on that. Bear in mind that 1mph = 18" per SECOND. 1mph is not uncommon for the amazon river system. 1 gallon in that size tank is going to be .5 - .75" of length on that tank depending on substrate depth. I'll let you do that math but seriously consider how much flow you're going to have in there. Even on the high end of 15x tank turnovers/hour you're nowhere near that kind of flow. Of course flow varies greatly depending on depth and plants but do bear in mind CO2 and flow are going to be critical. The subterrainean locline idea is one to seriously consider.
If you intend to keep discus you NEED to automate that water change method or at least find a way to speed it up. For example: My 40 gallon breeder tank takes a while to drain, 20-30 minutes using a python to the tub. It takes another 10-15 minutes to fill up. That's right on the edge of where I'm comfortable stopping the flow through a cannister filter. Depending on your plumbing you may not have this problem. If you can't make it a turn one valve to drain and turn another to fill kind of system, think hard about faster methods. The ten feet of 1" tubing into a 44 Gallon Rubbermaid Brute barrel, or 2, on wheels will vastly speed up your drain times. Fill times are another problem entirely. You can dump out the barrels at your leisure. I would not use these for filling the tank as there has been speculation about the plastics not necesarily being safe. Something about coral spermatazoa being killed by storage in one of these. I don't have the link handy but if you're just dumping that water it shouldn't matter.
Depending on what you buy for discus consider the following. Grow out any little ones in a separate tank. You'll be siphoning out uneaten food and doing a 50% WC DAILY if you want the biggest/best looking fish. Put that in a 125 gallon tank and the EI method and you're looking at a LOT of work. Plus all the food will get stuck in the gravel and plants requiring more maintenance. You could still dose macros one day and micros the next, but your doses are going to be much higher for EI. Most likely you'll be dosing one in the morning and one later on. Again, depends on what you want, just know that raising little discus into big discus takes a lot of work. You're better off with separate setups as learning plants is a pretty big project too. If you buy adults, then other than cost you're set.
I'll assume your APEX will do this already, but if not, for heaters I'd STRONGLY recomend one of the heater controllers as a final check in the system. Submersible types fail all the time. The glass ones are often a better option as you can look into them and see if there is any damage allowing water to get inside and mangle the thermostat. The hydor inlines are nice, but are limited in size. You may consider some of the DIY options ( if you're handy ) where people use much larger sizes of PVC pipes with larger inlet/outlets. With a compression fitting on one end they keep the bits that are likely to leak outside any of the water but still allow for one to pull out the heater for inspection if desired. Finnex has some heater controllers ( I don't know how good they are ) and Jehmco has some of the industrial kinds which may provide for better peace of mind. Use multiple heaters if possible so that in the even something fails on you can't parboil your tank. With a single heater running full tilt this is a distinct possibility. Remember that most heaters have a dirt cheap thermostat controlling them. Failures on or off are too common. You don't want to find out the hard way. If the APEX can control them you're good. Set the heaters to a few degrees higher than you want to go as an added safety. Also consider a leak/moisture detector for the APEX so it can contact you in the event that something leaks, assuming it can do this of course. Many of the reef controllers can.
You didn't mention anything about power. Chances are that your lights will be the biggest draw most of the time. Consider drawing out a complete electrical panel. You can just gang small 4x4 boxes together with GFCI in some and not others if needed. Having a switch to turn off the pumps and heaters and KNOW that everything in the tank is dead is a nice thing. Consider a grounding probe for the tank as well. With the GFCI it's dirt cheap insurance to know that you aren't going to get zapped when reaching into the tank. If you can run to separate circuits consider doing so with your equipment.
Also, if it's worth doing with heaters, it's worth considering two smaller pumps running in parallel. Or at least don't put everything on just one pump without at least considering extra water movement on a separate pump or powerhead/props in the tank. Your tank can survive for a few days without light and without heat. Chances are you'll be in trouble without water movement though. Even a simple airpump and air curtain in the back can be enough. Consider a second GFCI and outlet for JUST the pumps. A nuisance trip on a GFCI will kill everything downstream of it. You don't want to find out a GFCI went wonky and dropped everything because a pump spike weakened it. There are some reefers who only run GFCI on non pump items for this reason. Test any GFCI religiously. Your life depends on it.
Sorry to be the voice of doom here, but I'm running down all the scenarios in my head for my "ultimate" tank so it's a good exercise for me as well. I deal with server room environments so running two of everything to separate circuits and such is normal practice. It's all about risk tolerance and what you're willing to accept. You may feel it's all over the top and that's perfectly fine.
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