Well I recommend ditching the whole gravel and sand idea and going for a more 'dirted' tank- you can use potting soil with a plant substrate cap or you can use potting soil, a manufactured plant substrate like Eco-Complete or Fluval Stratum with a sand cap if you like the sandy look. This will give your plants the nutrients they will need but also you will need the right combo of ferts. Roots tabs and dry ferts are required IMO with a pressurized C02 system. For the C02 system, I would get a 5lb tank for yours (I have had my 5lb tank running on my 60g for almost 2 months and it hasn't ran out at all yet) or a 10lb tank- about $60. Or the regulator I use the AquaTek C02 regulator with integrated solenoid valve- $80. Then all you need is the diffuser, bubble counter + liquid, and the C02 tubing. I got all of this from GLA for $60, now, I got the top of the line items they had there (65mm atomic diffuser- $30; bubble counter w/check valve - )20 and 10 ft. Of C02 tubing- $7 plus a bottle of bubble counter liquid- $4). This is a great quality system for a fraction of the cost of a retail system (I found they run for around $400 to $500! Anything cheaper you can find may work well too, like you don't have to have a fancy diffuser like mine you can get that $5 ones they make that also works as well, but not as good. Back to ferts- are you using any? I use AquariumPlants root tabs TOTAL, PHOSPHATE and IRON and they work great, for dry ferts I dose PPS-Pro. I also dose Flourish Excel once in awhile as a C02 supplement and to combat algae. For lighting, if you want a lot of cool plants and a lot of plants in general, you lost likely will need something higher than a 2 bulb fixture. I run a 2 bulb T5HO fixture with 2 6700K 54watt T5HO bulbs inside as well as 2 48" TrueLumen Pro Series 8,000K LED light strips at 30watt each, which gets awesome PAR and wpg, so I am capable of growing the higher light plants which I am sure you would want if you want a heavier planted tank. Also higher light brings out a lot more color in the colored plants which IMO is a must!
For C02 are you looking at pressurized or DIY? I can give you good recommendations on a quality pressurized system that I also use for a nice price. Is the tank already planted? (The 55)
