It sounds like you have future plans that are more elaborate than the original set-up. I'm not familiar with the boat, so I wonder if it is wired for AC shore power. You don't mention it so I guess, probably not. Will you keep the boat at a slip with shore power? That may influence what you do.
Ward's reading materials are a must. You'll be able to answer a lot of your questions.
So you are basically starting from scratch with a battery. Just one deep cycle grp 24 or grp 27 will serve your immediate needs and you can use it to get your existing equipment and some new electronics up to speed. 2 x 6v golf cart batts wired in series are a very common way to generate 12v on a boat. Start out by determining the space you have for batteries and make sure the batts (or single batt) will fit in the space. Small boats usually have space limitations that can make battery selection problematic. Our battery space doesn't have the extra height that is required for golf cart batts, for instance.
Most sailors that use their boat for cruising usually have a primary bank of batteries, which may be 2 x 12v batts wired in parallel (for more capacity), AND an auxiliary battery that can be used for back-up in case the primary bank goes dead. Some of us use the auxiliary battery for a start battery as well. In this case, you may ultimately want up to 3 x 12v batteries (or 4 x 6v and 1 x 12v). Do you have the space? You will also need to consider the switching mechanisms that will be needed to operate your DC power. Do you have a battery switch on the boat now? If not, or if you are really starting from scratch, then plan your switching preference. There are different ways to skin a cat.
It is best to purchase batteries at one time so they all have similar use when you start. I don't think it would hurt for you to get a single battery to start out with and get everything operating. Then follow-up by purchasing your primary bank as your system grows. You can then use the first battery as your auxiliary.
I bring this up because you mention the desire to add features like pressurized water, electric flush, inverter, etc. The more gadgets you want, the more juice you will need … or an energizing source for recharging frequently.
This is where shore power comes in. I don't ever consider using an inverter because if I want AC power, I plug in at the dock. I use shore power for charging the batteries with a charger, water heater, and a few outlets that I have in the boat. You may find that all your charging needs are filled by solar. That's another good way to go. BTW, you won't be pressurizing the water tank … you can pressurize your water distribution with a pump, but your tank will most likely not be under pressure (while it does feed water to the pressure pump).