So for a self confessed tight arse trying to save money (something mistresses aren't very fond of), lets look at where you are upto.
O2 gas bottle $55, Reg $43, Airstone $23 Comes to over $120 Plus you will need some line hose clamps call it another $5 so you have spent $12, plus a couple of trips to various suppliers with more to come.
You still don't have a working system.
I suspect that the supplier has made that reg so it wont fit onto the oxygen bottle, for all the reasons listed in earlier posts, and a healthy dose of risk avoidance on their part. Looks like they aren't even stocking an O2 bottle (at least one compatible with that reg).
At a minimum you will be up for an adaptor, given you can find one that fits, I cant even find the adaptor Bunnings used to sell.
If you place much value on your time, and want a reasonably priced O2 system, just buy one from a reputable supplier, I doubt you will save much by doing a hell of a lot of running around, and at least you will get a working system with all the right parts, chosen for reliability and value for money when you look at the ongoing costs.
Mark