Hi Dominic,
Good hearing from you. The Hold 'N Treat system is available in two different configurations. The complete system DOES include the MSD and everything you need, except for the salt feed tank. You'd need the salt feed tank with pump, part # 32-3003. It comes complete with a 15 gallon holding tank. You could then get rid of your existing holding tank.
The other way you can do it, is to keep your existing 30 gallon holding tank and use that, then purchase the Hold 'N Treat controller, the MSD, and salt feed tank as individual components, then combine it all together.
What's the best way to go? I'm not sure. It depends mainly on where you're going to be using the boat. Just in the Bay, or out in the ocean, too?
Costwise, neither method has much of an advantage over the other. The main concern that I see at this point is the available space to mount the various components. Ideally, I'd like to see you forego the Electro-scan and use the Purasan with it, instead. With the Purasan, you wouldn't need a salt feed system, and would likely only have to replace the chlorine cartridge once a season, as the Purasan doesn't require salt or salt water. And, it can operate with either fresh or salt water.
Here's the "poop" on the Purasan:
http://www.raritaneng.com/pdf_files/purasan/L405v0804.pdf
Oh, by the way, you didn't mention how many and what make and model heads you have on the new boat? I'm guessing 2 (or maybe 3) VacuFlushes, but I don't know for certain.
It will require some planning, and give and take, back and forth, to come up with the best possible system for the boat. Oh, by the way, we have a new servicing dealer in Annapolis: Annapolis Harbor Boat Yard (formerly Steve's Yacht Repair). They're in Eastport, First street and the harbor, on the left. (410) 268-0092. Service Mgr.'s name is Kevin McPadden. But before all that, let's put our heads together and figure out which will be the best system to put on the boat.
Regards to Jacky and the kids.