The S9 is currently being replaced by the AirSense 10 (A10) series of machines. The A10 is probably as good as or better than the S9.
If you want the ResMed, you have to buy the ridiculously overpriced ResMed DC-DC converter. Look at the supplier list
websites for pictures and prices.
Other than the $100 or so cost, it's not that big a deal because it's similar to the power brick you have to use to power the machine on AC power. It does, in theory, protect your machine from over/under voltage and running your battery down.
The A10 DC-DC converter may be a little harder to find than the S9. I'm not sure whether the suppliers have them yet.
The PRS1 machines are easier to use because all you need is the DC cables.
Use caution with the power calculations. You need enough battery capacity to last you through however many days of cloudy weather you will experience. You also have to watch your power calculations. The solar panel may not be properly oriented to the sun while you're sailing. You also have to worry about whether the solar panel is shaded by the sail or other equipment.
Without the humidifier, you probably will only use 10 watts or so. My S9 uses only 30 watts average even with the humidifier.
Have you done calculations of sun hours, amp hours, etc. on your DC system? A "500 watt" solar system is only 500 watts for a few hours a day, and then only when it's sunny, and the panels are pointed to the right direction tilted to the south.
Get the humidifier. You may find you need it some times, and you might spend some nights not on the boat eventually. You might also get some cost advantage buying it up front because of insurance "discounts."
Be sure to properly calculate your costs with the deductible. If you do meet your deductible for the year, you "get back" your extra deductible later in the year because you meet your deductible sooner in the year. Also, "insurance prices" are often much cheaper than "non-insurance" ripoff prices.
ResMed says 6 months for the filters, but you might want more. Damp sea air might mess them up sooner. PRS1 6 months for the grey filter, 1 month for the white fine filters if you use them.
Masks are trickier. Plan on 6 months per mask, but some people find they have to replace their cushions on the mask more often. Spend as much time as you can while you're in port figuring out what mask works for you. You can buy replacement cushions online or from eBay or (maybe) Amazon and save money.
6 months per hose. In the wet environment, they may be a little more prone to needing replacement. Don't get the extra expensive heated hose if you're not using the humidifier. Heated hoses don't heat without a humidifier, and may not even fit. I buy authentic Respironics unheated hoses for $5 each delivered off of eBay. Pretty much all hoses interchange with each other.
Will you have some opportunity to order things to be shipped to you to pick up somewhere en route?
You might check craigslist while you're in Florida to see if you can find a cheap CPAP machine as a backup. You would hate to end up without CPAP out in the middle of the ocean.
What do you do for fresh water supply? The standard cleaning process can use a lot of water. With the humidity, you might want to get a bottle of Control III disinfectant in case germs starts growing on your mask and hoses. You don't need to use it often, but it might be good to know you can do a really good cleaning if you need it.
Read the useful links in my signature line below for hints on how to not get screwed with a bad CPAP machine by the DME (CPAP salesman).