Scott -
You can get a decent system for less money than that.
If you check enough of your local welding shops(not necessarily "welding supply shops".....there is a difference) you can probably find a 15 - 20lb. used bottle for $50 with gas in it.
Most places that re-fill bottles "swap" them. In other words, you come in with your empty bottle and they give you a full one, which will be used, for anywhere from $9 - 20. And most of the time it won't be aluminum.
There is no point in paying for a new bottle, especially a shiny new aluminum one for more money and then trading it for a used steel(not so shiny) one when you go to fill it.
There are places that fill on site, but in many cases you have to wait to get them filled.....sometimes for 5hrs....sometimes a day or two. While you're waiting for your gas, you pH is going up and the CO2 level in the aquarium is going down. Better to go in, set your oldie on the counter and pick up a full oldie and go home without losing a beat.
Just recently, I was looking for another bottle. I called around and found a welding shop that had an old 15 pounder with approx. 12lbs. of gas in it which I bought for $50. Look around before you drop more bucks than is necessary.
Here's a link to a needle valve that I've used for years and find them to be excellent:
http://www.clippard.com/store/display_details.asp?sku=MNV-2
If you buy it from Clippard through the mail it will cost you almost $20 with shipping. If you click on Clippard's Distributors list on the left they will give you the addresses of a place local to you that will sell you the valve for $4 - $5. Hmmm.....the choices!!!
I also have years of experience with this regulator:
http://rapidswholesale.net/Merchant2/merchant.mv?Screen=PROD&Product_Code=9T852
It's a nice reg. for the money.
Add to that a power head like the Aqua Clear Richer refers to a in his response, for a reactor, and some CO2 resistant tubing, and you're in business.
Len