Hi Dan, windscreen washer pumps are the impeller type, a very useful attribute and at a full 12volts will pump at 20-22 psi. In a sealed ballast tank system this will fill your tank to a level more than half of the tank volume. Sealed tank systems are exceedingly reliable and pretty foolproof if put together properly. One good aspect of this system is that you can't get the boat to submerge with a dwindling battery, the pump won't run fast enough to produce sufficient pressure to fill the tank so you know when it's time to pack up and go home! There is little to wear out with this system and it's pretty maintenence free so you will have little or no fidding at the pondside.
Remember,if you decide to go for it, ensure you make your ballast tank properly and of robust construction as at full pressure you will have 20 psi in there.
I would recommend to go for a static build every time, if you build it as a dynamic type you will have to ballast the boat down so heavily it will be decks awash when surfaced and you won't see anything of it when surface running. You will also find you are running around with no option but full power when dived so it will limit your outings due to current drain. There's nothing quite like watching your boat surface and sit proudly out of the water as all the ballast drains out.
Lucky feller, nice big boat, lots of room to work inside, it'll look impressive when it's done and even more so when it's skimming along below the surface of a clear pond.
Good luck with your future project.