I would recommend dual overflow boxes. At 120 gallons plus a 50 gallon sump you're going to want about 600-850 gallons per hour running through the sump system, so 2 overflow boxes rated at 500-800
GPH should work (2 would be for redundancy and extra surface skimming ability. Just put them on both sides, left and right, and you should not have any surface film and if one happens to lose suction, the other one can maintain it til you fix the problem). I'd recommend a Mag 12 as the return pump because at a 4' head (the height the hose runs from the pump over the back of the tank) it should be around the 600-700 gph flow rate.
A sump can be as big as you like it, and some tanks even have sumps bigger than the display itself. You need to decide what equipment you want in the tank and then make sure you design the sump that will house that equipment. You don't want 8" baffles if your
skimmer is designed for 10" of water. For now, you could go ahead and set the tank up with sand (decide on a shallow 1" sand bed or a deep 4-5" sand bed),
live rock (150-300lbs would be sufficient for a reef) and water (use
RO/DI mixed with salt). You can use the
HOB filters after a good bleach cleaning until you are ready to set up a reef. This would allow you to get comfortable with stuff like measuring S.G. and doing water changes while you research reef equipment.
What are the dimensions of the 120? Is it 6' long or 4' long? As the height will affect your lighting options.
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