hi, dont know about the system you are looking at, i just bought a dennerle profi 2000 which is discontinued now but it used to be $600, i got it for $95 on ebay uk (estimated £= $) This system is excellent and i have significant growth after just 2 days and the diffusor seems to be about 95% efficient, no bubbles leave the diffusor.
As for gas, the cheapest way is to buy food grade co2, get friendly with your local bar manager as it is used to drive the draught beer and soft drinks, mine was £6 / $10 for a 10kg bottle that will last a year. Failing that welding co2 from the hardware stores does the same job at half the LPS prices.
Good luck, im pretty new to this and i found adding co2 to be the most confusing part of starting my planted tank, but it really isnt as confusing as it seems.
I would just add that im glad i bought a whole system in one go then just add bits as you feel, but thats just me...