You need to hurry. The only Siberians left are the ones in your dealer's stock. I have none left and they are not making them in 2005. So get going!
Yes, this spa will serve you well. I have sold many of them, with great results.
The Freshwater stuff is simply the private labled chems which are not signigicantly different from other brands.
The Freshwater III system is an ozone system which allows you to soak chlorine free. It is constantly putting ozone into the water, 24 hours a day. You would put in a non-chlorine shock with each use. Some folks can get by with putting this 'MPS' in once a week, but you mentioned heavy use, so I think you may need it with each use.
You do put in some chlorine, but only as a monthly shock, or after a crowd exits the tub. This dissipates in a day or so, and is usually so low by the next use you don't really notice it. If that sounds good, ask the dealer to install one on your spa: it should be around $400 give or take.