I agree that you'll probably want some form of mechanical filtration. I would plumb the overflows to drain into 1 55G barrel that has 3 or 4 large (32") 100 to 200 micron filter socks. The more socks you have the less frequently they will clog and need to be cleaned. That should cover your mechanical filtration. Check out McMasterCarr or dudadiesel for socks. I would then plumb that barrel to a second barrel that contained your bio-media, your heaters and return pump. You could skip the barrels and try to find an old tank off craigslist or just use a large Rubbermaid stock tank.
It's always a good idea to run two smaller pumps versus one large pump. This redundancy is good insurance in case of pump failure; you'd be SOL if you were only running one pump and it fails, but if you were running two, the other could keep your system up and running. I would go with 2 Laguna Max-Flo 2900s. If you design the system with minimal head, you could make it really efficient. The less the pumps have to work to push water vertically, the more flow you can get with less watts. The Max-Flow 2900s will each do 2,245GPH @ 5' head on just 112 watts, so in total you'll have 4,490GPH on just 224 watts. I would use a minimum of 1.5" plumbing on the return lines for each pump.
The size of the heater required will depend in large part on the lowest room temps the tank will be in. If the difference in air temps and tank temps is 10F or less, you should be ok running 1 to 2 watts per gallon. If the difference is going to be greater than 10F, I would suggest running 3 to 4 watts per gallon. With a system as larger as yours, you will likely need a dedicated circuit(s) just for your heaters. Keep in mind that you don't want to draw more than 1,800 watts on a 20 Amp circuit (using the rule of thumb that you should not exceed 75% of the circuits max capacity). You absolutely do not want the heater to have to struggle for half the day to achieve your desired set temp. Overworking heaters is never a good idea. Based on the pic you posted, it looks like you have an HVAC duct that you could tap into which would help reduce the amount of wattage in aquarium heaters you'll need to heat the tank.
You should try to build fairly tight fitting lids to reduce evaporation and to help keep heat in the tank (which is basically one in the same). You may need to run a dehumidifier if the tank temps are going to be much higher than the room temps otherwise you might end up having issues with mold. Medium sized units draw ~300-500 watts whereas larger units can draw > 600+ watts. Just throwing that out there to keep in mind the number of dedicated circuits you may need.
I like your stocklist. Just a head's up that the Silver Dollars may eat your plants...the SDs I've had loved to munch on anything green (mine even chew on fake plastic plants).
Looking forward to seeing your build. Please post lots of pix =)
