You got it, i think what he means by ultraseals is what Deltec have, they screw and tighten an o ring around the tube, I have An arcadia end caps in the sumps, and they have been dunked countless times and still OK, but as before I prefer the Deltec.
On those HF ballasts, you should not find much difference between the double starter and the single, your link was for a single, save yerself more loot.