Welcome to PlantedTank.net,
Some people find that 55g are narrow and doesn't give much depth in the tank, but you can take a shot at trying to make it look deep. What kind of lighting do you have? what kind of reflector?
gH is basically the general hardness of your water, if you area planning to keep discus, they require soft water as well as clean, maybe 2x 50% water changes weekly.
kH measures amount of carbonate and bicarbonate ions in your tank, its oftenly used to used to caculate with pH to find the amount of Co2 that is in your tank.
You wouldn't want to have o2 tubes in your tank because they will quickly take away your co2, therefore you won't need it.
My pressurized co2 setup costs $125, if you can get it for cheaper, thats great if not you will not spend no where around $300 unless you are buying a ph controller and other equipment to set the co2 to where you like.
If you want a great guide, read Rex's as he stated above. I myself, find it helpful. But watch out, he has strong language!
ain't that right Rex?