I agree with fort. If you're converting the tank to sw in october, you'd be wasting money if you bought a co2 setup. Now if you are planning on keeping a planted tank and a sw tank too, then go ahead.
As far as difficulty, it's really not. It takes a little bit to understand how it works, but I'm 3 weeks in to my planted tank, about 2 weeks with co2, and I completely understand it now.
I think all in all, I have about $150 in my co2 setup. Thats the cylinder, regulator, reactor, and drop checker.