Josh, How much did you want to spend?
A 20 long is 12" wide, correct? You are going to need a skimmer with a small footprint. You also want to choose a skimmer rated for 1.5-2 x your total system volume.
Something like this would be what I would choose-
Bubble Magus BM-NAC6 100-160 Gallons
I would drill the tank and use one of these-
700 gph Overflow complete kit - Glass-Holes.com dope aquarium stuff
I would use 2 heaters. Redundancy is good with heaters. The theory is, if one fails, your tank won't die before you figure it out.
They are thermostatically controlled so they shouldn't constantly run and use a lot of electric.
Power heads- Decent power heads but not too expensive are Hydor koralias, Seio prop pumps, Aqueon.
Very expensive are vortec.
I would get two 1400gph power heads, give or take.
If you decide to drill the tank and use that overflow I linked, you'll want a "quiet one 3000" or a "Danner mag drive 7" for a return pump.
I wouldn't go much larger than that with a sump that size.