First off if you use a 55 you will only be holding 25 to 30 gallons in it.
Plexi off say 12" on the left side leaving bottom 1/2 to 1" open then skip over1/2" and run another sheet from bottom to spill over level. Now on the small compartment you just built wall off back 5" with 2 holes drilled for bulkhead fittings. Now attach 1" strips around perimeter of this chamber
that are 1/4" thick at least. There level should be just higher than the spill over wall you built. That wall will determine depth of water you run in the sump. Now using either eggcrate or perforated plexi make a shelf piece to sit on the ledge you built. This shelf will hold up the blue filter material for straining water. Now cut and attach 2 pvc pipe sections to bulk head fittings
drill holes about every inch down there lengthd, cap off ends, drill 2 air holes on top (straight over your line of holes). These will deliver water down to be strained. Then it will drip down (picking up oxygen). spill over into the refugium section. Now the opposite end has to house return pump, Use a submersable one, a skimmer (which can be a over the back or submersable model), heater, which will probably only run evenings when your lights are out), intake and outtake to a cannister filter (IMO run carbon). Once you have decided how much room this will take make your double spill over wall at this end. Whatever room you leave in the middle will be your refugium area.
This is just the configuration I found to use. It makes maintanance easy and
gives ease of access to entire sump system.
As far as weight goes, I have a 210 g display and a 45 g sump - the entire system only hold 200 gallons of water. So best guess with dispacement figured in for your system (sand, rock, and equipment) I would guess 95 to 100 gallons max. Keeping the system next to a wall that spans floor joices instead of parralleling them and you should be fine.